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/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