ci(test): faster mdx ci runtime

[rnbguy]

• Runs jobs in docker container https://docs.github.com/en/actions/using-jobs/running-jobs-in-a-container
• Uses pre-built https://github.com/rnbguy/mdx-docker
• GitHub workflow mounts standard directories in non-standard places in docker, which messes up with some github-actions
• macOS runner is disabled because it doesn't support docker

[rnbguy]

hey @shonfeder, I would love to have your feedback on this. This container solution works alright, except for a few hiccups which, once you know, are easy to fix. but mainly, I don't like that I am giving up on macOS runners.

What can be a good solution here? Pre-built ocaml-mdx binaries would be awesome. but I had a hard time building a statically linked binary.