Test ghcr.io as an alternate container registry and update necessary permissions

[vkt1414]

• ghcr.io may serve as an alternate container registry should dockerhub change free limits in the future
• Currently dockerhub limits to 100 downloads per IP in 6 hours, which we will never exceed as different VMs are used in parallel
• As far as I can tell from github's documentation, there are no limits for a public ghcr.io registry.
• Maintenance for ghcr.io will not be an overhead as the same gh actions script is used to build and push images to dockerhub and ghcr.io
• speeds are comparable.

By default, ghcr.io registries are private, and we must make them public from the settings. While private images have a difficult authentication process, public images are similar to dockerhub. Thoughts?

[vkt1414]

The images can be published on ghcr.io as well but won't be public. We will think about making them public when the need arises.