ghcr.io globs do not work (API mismatch)

The use of globs for ghcr.io registry images is not currently functional (see #496). In dockerhub.py, the assumption is for docker registry API calls, which fail for ghcr.io where the GitHub API should be used. It may be possible to use the legacy docker.pkg.github.com (which uses the docker registry API), but i am not sure if newer containers still get published there.