[From @joshdata: This is part of @toilal's work in #377 and https://github.com/Toilal/mailinabox/commit/1eb77c332bca0a74a37f792460903a1fb660befc. 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.

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