Prune healthcheck containers

If a deployment is interrupted it could leave stale healthcheck
containers around that prevent dependent images from being pruned.

test/cli/prune_test.rb

@@ -18,7 +18,8 @@ class CliPruneTest < CliTestCase
   test "containers" do
     run_command("containers").tap do |output|
       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
-    end
+      assert_match /docker container prune --force --filter label=service=healthcheck-app on 1.1.1.\d/, output
+     end
   end
 
   private

test/commands/prune_test.rb

@@ -20,10 +20,16 @@ class CommandsPruneTest < ActiveSupport::TestCase
       new_command.tagged_images.join(" ")
   end
 
-  test "containers" do
+  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.containers.join(" ")
+      new_command.app_containers.join(" ")
+  end
+
+  test "healthcheck containers" do
+    assert_equal \
+      "docker container prune --force --filter label=service=healthcheck-app",
+      new_command.healthcheck_containers.join(" ")
   end
 
   private

test/configuration_test.rb

@@ -124,6 +124,10 @@ class ConfigurationTest < ActiveSupport::TestCase
     assert_equal "app-missing", @config.service_with_version
   end
 
+  test "healthcheck service" do
+    assert_equal "healthcheck-app", @config.healthcheck_service
+  end
+
   test "env with missing secret" do
     assert_raises(KeyError) do
       config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!({