Support pushing Docker images to GitHub image registry

[MLenterman]

Would be nice if the image registry target(GHCR, DockerHub) can be automatically selected based on the image-repo-name. For example:

• image-repo-name "ghcr.io/wearefrank" would target GHCR
• image-repo-name "docker.io/wearefrank" would target DockerHub
• image-repo-name "wearefrank" would default to GHCR???

I think GHCR would make a more sensible default than DockerHub, because DockerHub always requires secrets to be configured, while GHCR can use the default GitHub provided GH_TOKEN/GITHUB_TOKEN if permission is configured in repo setting.

image-repo-name is used by multiple resuable workflows and GitHub actions in this ci-cd-templates repository. They currently specifically require the ghcr.io/docker.io prefix to NOT be included. Most of the work will be to change those workflows/actions to handle image-repo-name with a registry prefix.

[philipsens]

I also think ghcr is more logical, because multiple people can manage the packages instead of only 3 (2 excluding the bot account).