Publish images to GitHub container registry

No official decision is being made, but we need to start publishing images to both DockerHub and the new GitHub Container Registry. A decision will be made later on what registry we will continue to use, given DockerHub's new policy of pruning inactive images.

No files, tests, or entry points are named. Start by locating the existing DockerHub image-publishing workflow and its release configuration, then determine where GitHub Container Registry publishing belongs. Done means images are published to both registries without breaking the current release process; the issue notes that the long-term registry choice remains undecided.

Written by the indexing model from the issue text.