Move images to GitHub container registry

[dustinswales]

Issue

As part of #218, the images used for the containerized CI are stored on my (dustinswales) Dockerhub account.
We should move these to using the GitHub container registry, which is associated with the ufs-community organization

Solution

Set up correct permissions (e.g., GITHUB_TOKEN) and modify bld_mpas_images.yml to push to ghcr.io.