Simplify developer testing pipeline

The method and instructions used to run the tests via docker on a developers machine were a bit convoluted, requireing mounting various files in from the parent machine. Simplify by switching to usermod/groupmod which change the id of the somebody user inside to container to match the userid that owns the source code. This achieves the same result, allowing tox to write to the mounted /srv/app directory, but in a way that is simpler to invoke.

While here add a bash command line option which drops the user into a shell with the setup routines already run.