Dockerfile with cross-compiler targeting Windows x64

[Jacob Carlborg]

I've created a Dockerfile [1] containing LDC, configured for 
cross-compiling targeting Windows x64.

It's based on the instructions provided by kinke here [2].

Note, it downloads the MSVC libs from Dropbox.

[1] 
https://github.com/jacob-carlborg/docker-ldc-windows/blob/master/Dockerfile
[2] 
https://github.com/ldc-developers/ldc/pull/2142#issuecomment-304472412

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/Jacob Carlborg