Validate the build image

Kamal needs images to have the service label so it can track them for
pruning. Images built by Kamal will have the label, but externally built
ones may not.

Without it images will build up over time. The worst case is an outage
if all the hosts disks fill up at the same time.

We'll add a check for the label and halt if it is not there.

test/commands/builder_test.rb

@@ -103,6 +103,10 @@ class CommandsBuilderTest < ActiveSupport::TestCase
       builder.push.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
+
   private
     def new_builder_command(additional_config = {})
       Kamal::Commands::Builder.new(Kamal::Configuration.new(@config.merge(additional_config), version: "123"))