feat(docker,mk): add a Docker container wrapping the nix environment

[safl]

I wanted to create this as a DRAFT PR, as it is only for input to our offline discussion, however, for some reason that is currently not possible. The point of the docker-image:

• Show how to run this nix environment (misc. experimental features enabled)
• Provide a stepping stone for Docker-users
• Provide a means to running the nixenv in environments where Docker is an option and installing nix is not

The goal is to as the Makefile does, provide a "make docker" which should just drop into nix develop .#, thus providing the same development environment, as provided by nix, but via docker.

However, this does not work. For some reason running nix develop .# -c true fails.

[safl]

@jwdevantier thanks for fixing this up! It now fails due to the docs-check failing.

[safl]

I took the liberty of re-writing history because it looked so very wrong and I could not make sense of the changes made.

[safl]

Ah, the make docs check fails because linkcheck fails... because the repository is not publicly available....