diff --git a/lib/kamal/cli/accessory.rb b/lib/kamal/cli/accessory.rb
index 47bc1a06..b9cfee1f 100644
--- a/lib/kamal/cli/accessory.rb
+++ b/lib/kamal/cli/accessory.rb
@@ -5,11 +5,11 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
       if name == "all"
         KAMAL.accessory_names.each { |accessory_name| boot(accessory_name) }
       else
-        with_accessory(name) do |accessory|
+        with_accessory(name) do |accessory, hosts|
           directories(name)
           upload(name)
 
-          on(accessory.hosts) do
+          on(hosts) do
             execute *KAMAL.registry.login if login
             execute *KAMAL.auditor.record("Booted #{name} accessory"), verbosity: :debug
             execute *accessory.run
@@ -22,8 +22,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "upload [NAME]", "Upload accessory files to host", hide: true
   def upload(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           accessory.files.each do |(local, remote)|
             accessory.ensure_local_file_present(local)
 
@@ -39,8 +39,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "directories [NAME]", "Create accessory directories on host", hide: true
   def directories(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           accessory.directories.keys.each do |host_path|
             execute *accessory.make_directory(host_path)
           end
@@ -55,8 +55,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
       if name == "all"
         KAMAL.accessory_names.each { |accessory_name| reboot(accessory_name) }
       else
-        with_accessory(name) do |accessory|
-          on(accessory.hosts) do
+        with_accessory(name) do |accessory, hosts|
+          on(hosts) do
             execute *KAMAL.registry.login
           end
 
@@ -71,8 +71,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "start [NAME]", "Start existing accessory container on host"
   def start(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           execute *KAMAL.auditor.record("Started #{name} accessory"), verbosity: :debug
           execute *accessory.start
         end
@@ -83,8 +83,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "stop [NAME]", "Stop existing accessory container on host"
   def stop(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           execute *KAMAL.auditor.record("Stopped #{name} accessory"), verbosity: :debug
           execute *accessory.stop, raise_on_non_zero_exit: false
         end
@@ -107,8 +107,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
     if name == "all"
       KAMAL.accessory_names.each { |accessory_name| details(accessory_name) }
     else
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) { puts capture_with_info(*accessory.info) }
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) { puts capture_with_info(*accessory.info) }
       end
     end
   end
@@ -117,7 +117,7 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   option :interactive, aliases: "-i", type: :boolean, default: false, desc: "Execute command over ssh for an interactive shell (use for console/bash)"
   option :reuse, type: :boolean, default: false, desc: "Reuse currently running container instead of starting a new one"
   def exec(name, cmd)
-    with_accessory(name) do |accessory|
+    with_accessory(name) do |accessory, hosts|
       case
       when options[:interactive] && options[:reuse]
         say "Launching interactive command with via SSH from existing container...", :magenta
@@ -129,14 +129,14 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
 
       when options[:reuse]
         say "Launching command from existing container...", :magenta
-        on(accessory.hosts) do
+        on(hosts) do
           execute *KAMAL.auditor.record("Executed cmd '#{cmd}' on #{name} accessory"), verbosity: :debug
           capture_with_info(*accessory.execute_in_existing_container(cmd))
         end
 
       else
         say "Launching command from new container...", :magenta
-        on(accessory.hosts) do
+        on(hosts) do
           execute *KAMAL.auditor.record("Executed cmd '#{cmd}' on #{name} accessory"), verbosity: :debug
           capture_with_info(*accessory.execute_in_new_container(cmd))
         end
@@ -150,12 +150,12 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   option :grep, aliases: "-g", desc: "Show lines with grep match only (use this to fetch specific requests by id)"
   option :follow, aliases: "-f", desc: "Follow logs on primary server (or specific host set by --hosts)"
   def logs(name)
-    with_accessory(name) do |accessory|
+    with_accessory(name) do |accessory, hosts|
       grep = options[:grep]
 
       if options[:follow]
         run_locally do
-          info "Following logs on #{accessory.hosts}..."
+          info "Following logs on #{hosts}..."
           info accessory.follow_logs(grep: grep)
           exec accessory.follow_logs(grep: grep)
         end
@@ -163,7 +163,7 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
         since = options[:since]
         lines = options[:lines].presence || ((since || grep) ? nil : 100) # Default to 100 lines if since or grep isn't set
 
-        on(accessory.hosts) do
+        on(hosts) do
           puts capture_with_info(*accessory.logs(since: since, lines: lines, grep: grep))
         end
       end
@@ -192,8 +192,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "remove_container [NAME]", "Remove accessory container from host", hide: true
   def remove_container(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           execute *KAMAL.auditor.record("Remove #{name} accessory container"), verbosity: :debug
           execute *accessory.remove_container
         end
@@ -204,8 +204,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "remove_image [NAME]", "Remove accessory image from host", hide: true
   def remove_image(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           execute *KAMAL.auditor.record("Removed #{name} accessory image"), verbosity: :debug
           execute *accessory.remove_image
         end
@@ -216,8 +216,8 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   desc "remove_service_directory [NAME]", "Remove accessory directory used for uploaded files and data directories from host", hide: true
   def remove_service_directory(name)
     mutating do
-      with_accessory(name) do |accessory|
-        on(accessory.hosts) do
+      with_accessory(name) do |accessory, hosts|
+        on(hosts) do
           execute *accessory.remove_service_directory
         end
       end
@@ -227,7 +227,7 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
   private
     def with_accessory(name)
       if accessory = KAMAL.accessory(name)
-        yield accessory
+        yield accessory, accessory_hosts(accessory)
       else
         error_on_missing_accessory(name)
       end
@@ -240,4 +240,12 @@ class Kamal::Cli::Accessory < Kamal::Cli::Base
         "No accessory by the name of '#{name}'" +
         (options ? " (options: #{options.to_sentence})" : "")
     end
+
+    def accessory_hosts(accessory)
+      if KAMAL.specific_hosts&.any?
+        KAMAL.specific_hosts & accessory.hosts
+      else
+        accessory.hosts
+      end
+    end
 end
diff --git a/lib/kamal/cli/prune.rb b/lib/kamal/cli/prune.rb
index 236c7d55..498e4ec4 100644
--- a/lib/kamal/cli/prune.rb
+++ b/lib/kamal/cli/prune.rb
@@ -18,12 +18,16 @@ class Kamal::Cli::Prune < Kamal::Cli::Base
     end
   end
 
-  desc "containers", "Prune all stopped containers, except the last 5"
+  desc "containers", "Prune all stopped containers, except the last n (default 5)"
+  option :retain, type: :numeric, default: nil, desc: "Number of containers to retain"
   def containers
+    retain = options.fetch(:retain, KAMAL.config.retain_containers)
+    raise "retain must be at least 1" if retain < 1
+
     mutating do
       on(KAMAL.hosts) do
         execute *KAMAL.auditor.record("Pruned containers"), verbosity: :debug
-        execute *KAMAL.prune.app_containers
+        execute *KAMAL.prune.app_containers(retain: retain)
         execute *KAMAL.prune.healthcheck_containers
       end
     end
diff --git a/lib/kamal/cli/templates/deploy.yml b/lib/kamal/cli/templates/deploy.yml
index 20bbe71a..af018554 100644
--- a/lib/kamal/cli/templates/deploy.yml
+++ b/lib/kamal/cli/templates/deploy.yml
@@ -77,6 +77,10 @@ registry:
 # Bridge fingerprinted assets, like JS and CSS, between versions to avoid
 # hitting 404 on in-flight requests. Combines all files from new and old
 # version inside the asset_path.
+#
+# If your app is using the Sprockets gem, ensure it sets `config.assets.manifest`.
+# See https://github.com/basecamp/kamal/issues/626 for details
+#
 # asset_path: /rails/public/assets
 
 # Configure rolling deploys by setting a wait time between batches of restarts.
diff --git a/lib/kamal/commands/builder/base.rb b/lib/kamal/commands/builder/base.rb
index 8723fa45..f710807c 100644
--- a/lib/kamal/commands/builder/base.rb
+++ b/lib/kamal/commands/builder/base.rb
@@ -3,7 +3,7 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
   class BuilderError < StandardError; end
 
   delegate :argumentize, to: Kamal::Utils
-  delegate :args, :secrets, :dockerfile, :local_arch, :local_host, :remote_arch, :remote_host, :cache_from, :cache_to, to: :builder_config
+  delegate :args, :secrets, :dockerfile, :local_arch, :local_host, :remote_arch, :remote_host, :cache_from, :cache_to, :ssh, to: :builder_config
 
   def clean
     docker :image, :rm, "--force", config.absolute_image
@@ -14,7 +14,7 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
   end
 
   def build_options
-    [ *build_tags, *build_cache, *build_labels, *build_args, *build_secrets, *build_dockerfile ]
+    [ *build_tags, *build_cache, *build_labels, *build_args, *build_secrets, *build_dockerfile, *build_ssh ]
   end
 
   def build_context
@@ -60,6 +60,10 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
       end
     end
 
+    def build_ssh
+      argumentize "--ssh", ssh if ssh.present?
+    end
+
     def builder_config
       config.builder
     end
diff --git a/lib/kamal/commands/prune.rb b/lib/kamal/commands/prune.rb
index f9f37b24..40f7dfc4 100644
--- a/lib/kamal/commands/prune.rb
+++ b/lib/kamal/commands/prune.rb
@@ -13,10 +13,10 @@ class Kamal::Commands::Prune < Kamal::Commands::Base
       "while read image tag; do docker rmi $tag; done"
   end
 
-  def app_containers(keep_last: 5)
+  def app_containers(retain:)
     pipe \
       docker(:ps, "-q", "-a", *service_filter, *stopped_containers_filters),
-      "tail -n +#{keep_last + 1}",
+      "tail -n +#{retain + 1}",
       "while read container_id; do docker rm $container_id; done"
   end
 
diff --git a/lib/kamal/commands/traefik.rb b/lib/kamal/commands/traefik.rb
index c6ec8c8d..bb8f048b 100644
--- a/lib/kamal/commands/traefik.rb
+++ b/lib/kamal/commands/traefik.rb
@@ -1,7 +1,7 @@
 class Kamal::Commands::Traefik < Kamal::Commands::Base
   delegate :argumentize, :optionize, to: Kamal::Utils
 
-  DEFAULT_IMAGE = "traefik:v2.9"
+  DEFAULT_IMAGE = "traefik:v2.10"
   CONTAINER_PORT = 80
   DEFAULT_ARGS = {
     'log.level' => 'DEBUG'
diff --git a/lib/kamal/configuration.rb b/lib/kamal/configuration.rb
index 3fb87124..04c870a8 100644
--- a/lib/kamal/configuration.rb
+++ b/lib/kamal/configuration.rb
@@ -127,6 +127,10 @@ class Kamal::Configuration
     raw_config.require_destination
   end
 
+  def retain_containers
+    raw_config.retain_containers || 5
+  end
+
 
   def volume_args
     if raw_config.volumes.present?
@@ -218,7 +222,7 @@ class Kamal::Configuration
 
 
   def valid?
-    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version && ensure_valid_service_name
+    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version && ensure_retain_containers_valid && ensure_valid_service_name
   end
 
   def to_h
@@ -297,6 +301,12 @@ class Kamal::Configuration
       true
     end
 
+    def ensure_retain_containers_valid
+      raise ArgumentError, "Must retain at least 1 container" if retain_containers < 1
+
+      true
+    end
+
 
     def role_names
       raw_config.servers.is_a?(Array) ? [ "web" ] : raw_config.servers.keys.sort
diff --git a/lib/kamal/configuration/accessory.rb b/lib/kamal/configuration/accessory.rb
index 8fdcd227..d26c6c85 100644
--- a/lib/kamal/configuration/accessory.rb
+++ b/lib/kamal/configuration/accessory.rb
@@ -8,7 +8,7 @@ class Kamal::Configuration::Accessory
   end
 
   def service_name
-    "#{config.service}-#{name}"
+    specifics["service"] || "#{config.service}-#{name}"
   end
 
   def image
diff --git a/lib/kamal/configuration/builder.rb b/lib/kamal/configuration/builder.rb
index c7d81922..9daa3138 100644
--- a/lib/kamal/configuration/builder.rb
+++ b/lib/kamal/configuration/builder.rb
@@ -81,6 +81,10 @@ class Kamal::Configuration::Builder
     end
   end
 
+  def ssh
+    @options["ssh"]
+  end
+
   private
     def valid?
       if @options["cache"] && @options["cache"]["type"]
diff --git a/lib/kamal/sshkit_with_ext.rb b/lib/kamal/sshkit_with_ext.rb
index eb8f359b..e0c62c3a 100644
--- a/lib/kamal/sshkit_with_ext.rb
+++ b/lib/kamal/sshkit_with_ext.rb
@@ -1,5 +1,6 @@
 require "sshkit"
 require "sshkit/dsl"
+require "net/scp"
 require "active_support/core_ext/hash/deep_merge"
 require "json"
 
diff --git a/test/cli/accessory_test.rb b/test/cli/accessory_test.rb
index c4cf4828..d8f3fc63 100644
--- a/test/cli/accessory_test.rb
+++ b/test/cli/accessory_test.rb
@@ -148,6 +148,30 @@ class CliAccessoryTest < CliTestCase
     assert_match "rm -rf app-mysql", run_command("remove_service_directory", "mysql")
   end
 
+  test "hosts param respected" do
+    Kamal::Cli::Accessory.any_instance.expects(:directories).with("redis")
+    Kamal::Cli::Accessory.any_instance.expects(:upload).with("redis")
+
+    run_command("boot", "redis", "--hosts", "1.1.1.1").tap do |output|
+      assert_match /docker login.*on 1.1.1.1/, output
+      refute_match /docker login.*on 1.1.1.2/, output
+      assert_match "docker run --name app-redis --detach --restart unless-stopped --log-opt max-size=\"10m\" --publish 6379:6379 --env-file .kamal/env/accessories/app-redis.env --volume $PWD/app-redis/data:/data --label service=\"app-redis\" redis:latest on 1.1.1.1", output
+      refute_match "docker run --name app-redis --detach --restart unless-stopped --log-opt max-size=\"10m\" --publish 6379:6379 --env-file .kamal/env/accessories/app-redis.env --volume $PWD/app-redis/data:/data --label service=\"app-redis\" redis:latest on 1.1.1.2", output
+    end
+  end
+
+  test "hosts param intersected with configuration" do
+    Kamal::Cli::Accessory.any_instance.expects(:directories).with("redis")
+    Kamal::Cli::Accessory.any_instance.expects(:upload).with("redis")
+
+    run_command("boot", "redis", "--hosts", "1.1.1.1,1.1.1.3").tap do |output|
+      assert_match /docker login.*on 1.1.1.1/, output
+      refute_match /docker login.*on 1.1.1.3/, output
+      assert_match "docker run --name app-redis --detach --restart unless-stopped --log-opt max-size=\"10m\" --publish 6379:6379 --env-file .kamal/env/accessories/app-redis.env --volume $PWD/app-redis/data:/data --label service=\"app-redis\" redis:latest on 1.1.1.1", output
+      refute_match "docker run --name app-redis --detach --restart unless-stopped --log-opt max-size=\"10m\" --publish 6379:6379 --env-file .kamal/env/accessories/app-redis.env --volume $PWD/app-redis/data:/data --label service=\"app-redis\" redis:latest on 1.1.1.3", output
+    end
+  end
+
   private
     def run_command(*command)
       stdouted { Kamal::Cli::Accessory.start([*command, "-c", "test/fixtures/deploy_with_accessories.yml"]) }
diff --git a/test/cli/prune_test.rb b/test/cli/prune_test.rb
index 77c3ebd4..11acc469 100644
--- a/test/cli/prune_test.rb
+++ b/test/cli/prune_test.rb
@@ -20,6 +20,15 @@ class CliPruneTest < CliTestCase
       assert_match /docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +6 | while read container_id; do docker rm $container_id; done on 1.1.1.\d/, output
       assert_match /docker container prune --force --filter label=service=healthcheck-app on 1.1.1.\d/, output
      end
+
+    run_command("containers", "--retain", "10").tap do |output|
+      assert_match /docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +11 | while read container_id; do docker rm $container_id; done on 1.1.1.\d/, output
+      assert_match /docker container prune --force --filter label=service=healthcheck-app on 1.1.1.\d/, output
+    end
+
+    assert_raises(RuntimeError, "retain must be at least 1") do
+      run_command("containers", "--retain", "0")
+    end
   end
 
   private
diff --git a/test/commands/accessory_test.rb b/test/commands/accessory_test.rb
index 687c7fe5..92ed1211 100644
--- a/test/commands/accessory_test.rb
+++ b/test/commands/accessory_test.rb
@@ -34,6 +34,7 @@ class CommandsAccessoryTest < ActiveSupport::TestCase
           ]
         },
         "busybox" => {
+          "service" => "custom-busybox",
           "image" => "busybox:latest",
           "host" => "1.1.1.7"
         }
@@ -57,7 +58,7 @@ class CommandsAccessoryTest < ActiveSupport::TestCase
       new_command(:redis).run.join(" ")
 
     assert_equal \
-      "docker run --name app-busybox --detach --restart unless-stopped --log-opt max-size=\"10m\" --env-file .kamal/env/accessories/app-busybox.env --label service=\"app-busybox\" busybox:latest",
+      "docker run --name custom-busybox --detach --restart unless-stopped --log-opt max-size=\"10m\" --env-file .kamal/env/accessories/custom-busybox.env --label service=\"custom-busybox\" busybox:latest",
       new_command(:busybox).run.join(" ")
   end
 
@@ -65,7 +66,7 @@ class CommandsAccessoryTest < ActiveSupport::TestCase
     @config[:logging] = { "driver" => "local", "options" => { "max-size" => "100m", "max-file" => "3" } }
 
     assert_equal \
-      "docker run --name app-busybox --detach --restart unless-stopped --log-driver \"local\" --log-opt max-size=\"100m\" --log-opt max-file=\"3\" --env-file .kamal/env/accessories/app-busybox.env --label service=\"app-busybox\" busybox:latest",
+      "docker run --name custom-busybox --detach --restart unless-stopped --log-driver \"local\" --log-opt max-size=\"100m\" --log-opt max-file=\"3\" --env-file .kamal/env/accessories/custom-busybox.env --label service=\"custom-busybox\" busybox:latest",
       new_command(:busybox).run.join(" ")
   end
 
diff --git a/test/commands/builder_test.rb b/test/commands/builder_test.rb
index 22ed8575..51bb837c 100644
--- a/test/commands/builder_test.rb
+++ b/test/commands/builder_test.rb
@@ -111,6 +111,14 @@ class CommandsBuilderTest < ActiveSupport::TestCase
       builder.push.join(" ")
   end
 
+  test "build with ssh agent socket" do
+    builder = new_builder_command(builder: { "ssh" => 'default=$SSH_AUTH_SOCK' })
+
+    assert_equal \
+      "-t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile --ssh default=$SSH_AUTH_SOCK",
+      builder.target.build_options.join(" ")
+  end
+
   test "validate image" do
     assert_equal "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:123 | grep -x app || (echo \"Image dhh/app:123 is missing the `service` label\" && exit 1)", new_builder_command.validate_image.join(" ")
   end
diff --git a/test/commands/prune_test.rb b/test/commands/prune_test.rb
index 00db4ccc..c4a56a9a 100644
--- a/test/commands/prune_test.rb
+++ b/test/commands/prune_test.rb
@@ -23,7 +23,11 @@ class CommandsPruneTest < ActiveSupport::TestCase
   test "app containers" do
     assert_equal \
       "docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +6 | while read container_id; do docker rm $container_id; done",
-      new_command.app_containers.join(" ")
+      new_command.app_containers(retain: 5).join(" ")
+
+    assert_equal \
+      "docker ps -q -a --filter label=service=app --filter status=created --filter status=exited --filter status=dead | tail -n +4 | while read container_id; do docker rm $container_id; done",
+      new_command.app_containers(retain: 3).join(" ")
   end
 
   test "healthcheck containers" do
diff --git a/test/configuration/accessory_test.rb b/test/configuration/accessory_test.rb
index 522dc631..4b56be51 100644
--- a/test/configuration/accessory_test.rb
+++ b/test/configuration/accessory_test.rb
@@ -49,6 +49,7 @@ class ConfigurationAccessoryTest < ActiveSupport::TestCase
           }
         },
         "monitoring" => {
+          "service" => "custom-monitoring",
           "image" => "monitoring:latest",
           "roles" => [ "web" ],
           "port" => "4321:4321",
@@ -72,6 +73,7 @@ class ConfigurationAccessoryTest < ActiveSupport::TestCase
   test "service name" do
     assert_equal "app-mysql", @config.accessory(:mysql).service_name
     assert_equal "app-redis", @config.accessory(:redis).service_name
+    assert_equal "custom-monitoring", @config.accessory(:monitoring).service_name
   end
 
   test "port" do
diff --git a/test/configuration/builder_test.rb b/test/configuration/builder_test.rb
index da30d915..4f4bbc40 100644
--- a/test/configuration/builder_test.rb
+++ b/test/configuration/builder_test.rb
@@ -148,4 +148,14 @@ class ConfigurationBuilderTest < ActiveSupport::TestCase
 
     assert_equal "..", @config_with_builder_option.builder.context
   end
+
+  test "ssh" do
+    assert_nil @config.builder.ssh
+  end
+
+  test "setting ssh params" do
+    @deploy_with_builder_option[:builder] = { "ssh" => 'default=$SSH_AUTH_SOCK' }
+
+    assert_equal 'default=$SSH_AUTH_SOCK', @config_with_builder_option.builder.ssh
+  end
 end
diff --git a/test/configuration_test.rb b/test/configuration_test.rb
index 37b5b4a0..c538a82a 100644
--- a/test/configuration_test.rb
+++ b/test/configuration_test.rb
@@ -319,4 +319,12 @@ class ConfigurationTest < ActiveSupport::TestCase
     end
     assert_match /bar isn't defined/, error.message
   end
+
+  test "retain_containers" do
+    assert_equal 5, @config.retain_containers
+    config = Kamal::Configuration.new(@deploy_with_roles.merge(retain_containers: 2))
+    assert_equal 2, config.retain_containers
+
+    assert_raises(ArgumentError) { Kamal::Configuration.new(@deploy_with_roles.merge(retain_containers: 0)) }
+  end
 end
diff --git a/test/integration/docker/deployer/app/config/deploy.yml b/test/integration/docker/deployer/app/config/deploy.yml
index ec070f6c..baedca99 100644
--- a/test/integration/docker/deployer/app/config/deploy.yml
+++ b/test/integration/docker/deployer/app/config/deploy.yml
@@ -24,9 +24,10 @@ traefik:
   args:
     accesslog: true
     accesslog.format: json
-  image: registry:4443/traefik:v2.9
+  image: registry:4443/traefik:v2.10
 accessories:
   busybox:
+    service: custom-busybox
     image: registry:4443/busybox:1.36.0
     cmd: sh -c 'echo "Starting busybox..."; trap exit term; while true; do sleep 1; done'
     roles:
diff --git a/test/integration/docker/deployer/setup.sh b/test/integration/docker/deployer/setup.sh
index a2c9c6e8..0cd511d9 100755
--- a/test/integration/docker/deployer/setup.sh
+++ b/test/integration/docker/deployer/setup.sh
@@ -19,7 +19,7 @@ push_image_to_registry_4443() {
 
 install_kamal
 push_image_to_registry_4443 nginx 1-alpine-slim
-push_image_to_registry_4443 traefik v2.9
+push_image_to_registry_4443 traefik v2.10
 push_image_to_registry_4443 busybox 1.36.0
 
 # .ssh is on a shared volume that persists between runs. Clean it up as the
diff --git a/test/integration/main_test.rb b/test/integration/main_test.rb
index a0364fb2..856781cb 100644
--- a/test/integration/main_test.rb
+++ b/test/integration/main_test.rb
@@ -32,7 +32,7 @@ class MainTest < IntegrationTest
     assert_match /Traefik Host: vm2/, details
     assert_match /App Host: vm1/, details
     assert_match /App Host: vm2/, details
-    assert_match /traefik:v2.9/, details
+    assert_match /traefik:v2.10/, details
     assert_match /registry:4443\/app:#{first_version}/, details
 
     audit = kamal :audit, capture: true
diff --git a/test/integration/traefik_test.rb b/test/integration/traefik_test.rb
index ed8a956f..6f735612 100644
--- a/test/integration/traefik_test.rb
+++ b/test/integration/traefik_test.rb
@@ -52,11 +52,11 @@ class TraefikTest < IntegrationTest
 
   private
     def assert_traefik_running
-      assert_match /traefik:v2.9   "\/entrypoint.sh/, traefik_details
+      assert_match /traefik:v2.10   "\/entrypoint.sh/, traefik_details
     end
 
     def assert_traefik_not_running
-      refute_match /traefik:v2.9   "\/entrypoint.sh/, traefik_details
+      refute_match /traefik:v2.10   "\/entrypoint.sh/, traefik_details
     end
 
     def traefik_details