Add Windows docker file
This is a Dockerfile I prepared for the GLib CI. During the hackfest I verified it is possible to compile gst-build inside it.
Note that Windows docker requires Windows host, I can provide one for testing (a spare x230 I installed with Windows HyperV Server 2016) but it will be too slow for production. Also docker-in-docker is impossible for Windows, so images will have to be built on the host and uploaded manually.