Allow non-root USER

Add sti-fake-user Dockerfile which extends sti-fake and runs as a
non-root user.

Add integration test to ensure that non-root images work ok.

Fix issues with support for non-root users:
- set config.User prior to committing if the base image has a USER
- workaround issue in Docker where you can't run a container from an
  image whose original container had a file bind mount, and you don't
  specify something to satisfy that file bind mount when running.