Add GitHub Action to publish MAP(s) on GHCR

[tomaroberts]

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.

[tomaroberts]

Relevant/reminder:

Could build a unit test which uses monai-deploy package to build the MAP, such as: https://github.com/Project-MONAI/monai-deploy-app-sdk/blob/main/tests/system/packager/test_packager.py