Host docker images on GitHub Container Registry

They're on DockerHub now. We might consider putting them onto ghcr because that service gives us statistics on downloads for each tag, rather then just all tags at once.

Review how the project currently publishes its Docker images to DockerHub, then check the GitHub Container Registry requirements for hosting the same images. The work is done when the images are available on ghcr and download statistics can be viewed separately for each tag.

Written by the indexing model from the issue text.