Remove GAP's DockerHub account

[wilfwilson]

The images at https://registry.hub.docker.com/u/gapsystem are very out of date now and should be removed. I think it would be best to remove the whole account, or at least to remove the images.

Who has access to this account? @alex-konovalov?

Instead, we can use GitHub's container registry, as we are already doing for the existing containers at e.g. https://github.com/gap-system/gap-docker-master/pkgs/container/gap-docker-master

[olexandr-konovalov]

Yes, me - will do