fix: inject alternate label when building from an image

This label is otherwise missing, causing tooling that might want to know about it's pre-existing labels - specifically, docker-image-labeler - to fail to run as expected.

tests/unit/git_4.bats

@@ -123,7 +123,7 @@ EOF
   echo "status: $status"
   assert_success
 
-  run /bin/bash -c "docker image inspect dokku/$TEST_APP:latest | grep 'alternate-tags'"
+  run /bin/bash -c "docker image inspect dokku/$TEST_APP:latest --format '{{ index .Config.Labels \"com.dokku.docker-image-labeler/alternate-tags\" }}'"
   echo "output: $output"
   echo "status: $status"
   assert_success