Absorb James's Docker setup

[wilfwilson]

@james-d-mitchell Has created several repositories related to the older gap-system Docker containers. He has been using them for the CI.

• https://github.com/james-d-mitchell/gap-docker-minimal-arm
• https://github.com/james-d-mitchell/gap-docker-minimal-arm-32
• https://github.com/james-d-mitchell/gap-docker-minimal-32
• https://github.com/james-d-mitchell/gap-docker-minimal
• https://github.com/james-d-mitchell/gap-docker

The images are hosted on Docker Hub, with various tags https://hub.docker.com/u/jamesdbmitchell

Let's absorb the things that he does differently. That way, James can retire his setup (in favour of using this repository), and everyone can benefit.