use rbenv+ruby-build instead of rvm

[flavorjones]

While working on https://github.com/rake-compiler/rake-compiler-dock/issues/75 I found that RVM has issues on Alpine, specifically https://github.com/rvm/rvm/issues/5255

Now, while we could probably use another musl-based image (and not Alpine specifically, since the RVM problem is really related to Alpine's use of busybox), I will point out that ruby-build appears to be more actively maintained than rvm, is simpler, and is a bit easier to work with. So I think this is a good change regardless (pending green tests).

Changes:

• user rvm no longer exists, replaced by rubyuser
• rvm has been replaced by rbenv and ruby-build
  • no longer applying the sendfile patches to Ruby
  • no longer updating gems belonging to the bootstrap rubies

Also in this PR, I've updated the predefined user and group lists to be the superset of all our images.

[flavorjones]

@larskanis are you OK with this change? can you let me know if you have concerns or want to get on a call to discuss it?

[larskanis]

Sorry @flavorjones for the delay! In general I'm fine with the change.

The sendfile patches were necessary for VirtualBox. I guess this is still the case, but VirtualBox emulation for docker is not very common these days. So we can remove them.

In any case: Great work!