Runit beta feedback

Now that !721 (merged) is merged, we have optional support for Runit in gitlab-development-kit. The goal is to replace Foreman with something more robust. What we don't know if the alternative is nice to work with. That is why we want your feedback!

Demo video: https://youtu.be/njukxpVM1L0

Beta documentation: https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/runit.md

The current state is usable but it will have rough edges. Finding those edges is one of the goals of this beta. The other goal is to find out if the new workflow (gdk start, gdk stop, gdk tail, no more gdk run) is acceptable.

This is a follow-up to #443 (closed) and