Migrate from Supervisor to S6 Overlay

[damianmoore]

Used by LinuxServer images. Smaller, faster, tidier. Cleans up after finished processes. Has extra utilities to deal with permissions problems and dropping permissions.

In the first instance I'd install this using their tarball installer as the base images will cause lots of compatibility issues with other things we need to run.

https://github.com/just-containers/s6-overlay https://tutumcloud.wordpress.com/2015/05/20/s6-made-easy-with-the-s6-overlay/ https://tutumcloud.wordpress.com/2014/12/02/docker-and-s6-my-new-favorite-process-supervisor/