Clean up and extend docker build instructions

[pasqu4le]

Clarification and motivation

We currently have a README in the docker directory, which explains how to build packages using docker.

This document is probably not wrong, but it is both lacking some options (especially from our scripts) as well as being disorganized (as it has been updated over time).

This should be extended with more information, updated to correct the instructions (if necessary) and cleaned up to be easier to understand.

Acceptance criteria

The docker build instructions are easy to understand and explain all available options