Updated documentation for Visual Studio Build Tools container

I’ve updated the documentation for building a Docker container image for Visual Studio Build tools based on recent feedback that managed code may fail to run. In the case of MSBuild, you might see an error like, C:\BuildTools\MSBuild\15.0\bin\Roslyn\Microsoft.CSharp.Core.targets(84,5): error MSB6003: The specified task executable “csc.exe” could not be run. Could not load file or assembly… Read more