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.

lib/kamal/cli/build.rb

@@ -50,6 +50,7 @@ class Kamal::Cli::Build < Kamal::Cli::Base
         execute *KAMAL.auditor.record("Pulled image with version #{KAMAL.config.version}"), verbosity: :debug
         execute *KAMAL.builder.clean, raise_on_non_zero_exit: false
         execute *KAMAL.builder.pull
+        execute *KAMAL.builder.validate_image
       end
     end
   end

lib/kamal/commands/builder.rb

@@ -1,7 +1,7 @@
 require "active_support/core_ext/string/filters"
 
 class Kamal::Commands::Builder < Kamal::Commands::Base
-  delegate :create, :remove, :push, :clean, :pull, :info, to: :target
+  delegate :create, :remove, :push, :clean, :pull, :info, :validate_image, to: :target
 
   def name
     target.class.to_s.remove("Kamal::Commands::Builder::").underscore.inquiry

lib/kamal/commands/builder/base.rb

@@ -21,6 +21,12 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
     config.builder.context
   end
 
+  def validate_image
+    pipe \
+      docker(:inspect, "-f", "'{{ .Config.Labels.service }}'", config.absolute_image),
+      [:grep, "-x", config.service, "||", "(echo \"Image #{config.absolute_image} is missing the `service` label\" && exit 1)"]
+  end
+
 
   private
     def build_tags

test/cli/build_test.rb

@@ -57,6 +57,7 @@ class CliBuildTest < CliTestCase
     run_command("pull").tap do |output|
       assert_match /docker image rm --force dhh\/app:999/, output
       assert_match /docker pull dhh\/app:999/, output
+      assert_match "docker inspect -f '{{ .Config.Labels.service }}' dhh/app:999 | grep -x app || (echo \"Image dhh/app:999 is missing the `service` label\" && exit 1)", output
     end
   end
 

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"))