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