Dockerfile.windows uses a deprecated base image

[devinvisible]

From https://blogs.msdn.microsoft.com/dotnet/2017/03/07/announcing-net-core-tools-1-0/#docker

We adopted changes to the Docker tag scheme we use at microsoft/dotnet. The “-projectjson” and “-msbuild” differentiation is now gone. The tags are significantly simpler, as you can see from the tags above.

[natemcmaster]

Closing as we have removed dockerfiles from this repo. See #812 for more details.