Adds the `-u` flag for running container with non-root privs

Xref

- https://github.com/bgruening/docker-ipython-notebook/pull/20/
- https://github.com/bgruening/docker-ipython-notebook/pull/17

This is all concerning infrastructure for "non-root containers". Here we add the `-u` flag for running as a non-root user. See PR20 for more discussion/information as to the rationale for `-u`.

`geteuid` used in case they're doing something stupid when running Galaxy. We run it under supervisord, and it looks like `uid==euid`, so leaving the `euid` call "just in case".