[From @joshdata: This is part of @toilal's work in #377 and 1eb77c332b. The changes are:
* Separates out the runit configuration from starting Mail-in-a-Box setup so that Mail-in-a-Box setup does not block the starting of runit services and we can assume that runit is running during setup (i.e. we can restart services).
* Adds a SKIP_INSTALL flag so that the container can be restarted without re-running the whole Mail-in-a-Box setup.
* Made containers/docker/run more flexible.
* I'm also adding some "|| exit 0"s to the run script to stop if there are any docker errors.
* I'm also adding the prereqs installs from questions.sh into Dockerfile so we don't have to reinstall each time.
]
Docker support was initially worked on in 2bbb7a5e7e, but it never really worked.
This extends f7d7434012800c3572049af82a501743d4aed583 which was an old branch for docker work.
