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.

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

@@ -299,7 +299,7 @@ class ConfigurationTest < ActiveSupport::TestCase
 
     assert_equal "alternate_web", config.primary_role
     assert_equal "1.1.1.4", config.primary_host
-    assert config.role(:alternate_web).primary? 
+    assert config.role(:alternate_web).primary?
     assert config.role(:alternate_web).running_traefik?
   end
 
@@ -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