Create Ruby Images

[fkorotkov]

To simplify cirruslabs/cirrus-ci-docs#245

[duckinator]

Now that you added msys2 to contrib/cmake, I think this may consist entirely of:

FROM cirrusci/windowsservercore:cmake

RUN choco install -y ruby --version INSERT_RUBY_VERSION_HERE
RUN ridk install 2 3

But I've not tested that. :)

A good starting point in terms of versions to provide images for is probably the Ruby versions that are in "normal maintenance", as listed on https://www.ruby-lang.org/en/downloads/branches/

That's usually 2-3 versions. Currently it's 2.5 + 2.6.

[duckinator]

I made an attempt at implementing this, but unfortunately I don't have a working Windows system and I can't get Docker working in a Windows VM.

It may provide a good starting point, though: duckinator@021ea05