Add preliminary Dockerfile for Windows 64 bit Docker image.

[insertinterestingnamehere]

Closes #275.

The next thing to do is try to set up a build worker that can use this Docker image.

Some things to be aware of:

• It's not clear to me whether we can distribute a docker image with the MSVC build tools in it. I believe that we'll just have to build the image from the Dockerfile on each worker and then tag it with the name we want to use.
• The C/C++ compilers for versions of Python older than 3.5 refuse to install on machines with only a Technical Preview or trial license of Windows. That prevents them from being usable inside the Docker image right now. Hopefully this changes when Windows Server 2016 is actually released.