[build2] Dockerfiles and Docker Images

[Boris Kolpackov]

Thanks for sharing these.

Just for my understanding, these are meant as a starting point
for someone wanting to create a container for CI, right? If
that's correct, then I am wondering why you've decided to setup
your own CI instead of using ci.cppget.org (not that there is
anything wrong with that). I am guessing this is due to some
limitations of our CI service (and can probably guess what the
are) but I would like to know your opinion/motivation.