feat(build): Add a Dockerfile for firmware building

Original work from @sistason in MR !121, rebased
on latest master.

Commit 1ea42e8eac0498a027f8c303e33a87f93df4dba5
Author: Kai <sistason@sistason.de>
Date:   Mon Aug 19 21:58:11 2019 +0200

    Adds Docker for firmware-building

    This commit adds a Dockerfile to build a docker
    container to build the firmware with.  It also
    updates the documentation on how to do that.