Publish images also to alternative docker registries

[shyim]

My GitHub pipeline failed due to:

▀  Pulling image: basecamp/kamal-proxy:v0.8.0 (0s)
                                                                                
  ERROR   toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit

It would be nice if the image would be also available at ghcr.io/quay.io whatever