Add instructions and code to create the docker image with nix as a container

[paolino]

Why

This will overcome the issue of installing nix locally if you need to build an unpublished docker image.

What

• Add a Dockerfile as in https://github.com/cardano-foundation/cardano-wallet/blob/62d79d224714e945935548c26d760e7177fa8a27/nix/Dockerfile#L26
• Add instructions on how to use it in installation/docker.md