Add GitHub Action to publish MAP(s) on GHCR

On release, it would be good to automatically build the latest container.

See: https://docs.github.com/en/actions/publishing-packages/publishing-docker-images#publishing-images-to-github-packages
[Edit... this might not be possible because requires `.Dockerfile` in root of repo, which we don't have because MAP built by running `monai-deploy package`]

NB: might need to implement something to cater for having to create `map-init` first.

Start with the GitHub Actions documentation linked in the issue and inspect how releases currently work. Determine how `monai-deploy package` builds the MAP without a root `.Dockerfile`, and whether `map-init` must be created first. Done means a release workflow builds the latest MAP(s) and publishes them to GHCR.

Written by the indexing model from the issue text.