Docker CI build

Possible docker build configurations:

Docker Windows:

• net40 (https://hub.docker.com/r/microsoft/dotnet-framework/)

Docker Linux:

• netstandard 1.1/2.0, with .net core sample apps https://github.com/dotnet/dotnet-docker/blob/master/samples/dotnetapp/Dockerfile. Those targets include all Unity builds (netstandard2.0)
• Android https://github.com/nathansamson/xamarin-android-docker/blob/master/Dockerfile (probably tricky, unofficially supported) https://github.com/xamarin/xamarin-android/blob/master/Dockerfile
• iOS, Mac: No.

https://github.com/dotnet/dotnet-docker

https://github.com/dotnet/dotnet-docker/blob/master/samples/dotnetapp/Dockerfile