Config the number of containers to keep

By default we keep 5 containers around for rollback. The containers
don't take much space, but the images for them can.

Make the number of containers to retain configurable, either in the
config with the `retain_containers` setting on the command line
with the `--retain` option.

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

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
 

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_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version && ensure_retain_containers_valid
   end
 
   def to_h
@@ -291,6 +295,12 @@ 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

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

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

test/configuration_test.rb

@@ -309,4 +309,12 @@ 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