Cirrus CI got some optimizations for containers based of `microsoft/windowsservercore:latest`.
Now startup time for windows builds is around 1:30 seconds instead of around 4 minutes.
to: @aslushnikov
This patch adds experimental support for Cirrus CI builds.
Cirrus CI has both Windows and Linux, uses google cloud and runs builds
in Docker containers.
This seems to be a promising combination for our needs.