Set up iso building in Docker for MSVC and GCC

That will allow us to use one big server to build many configurations (even at the same time), Buildbot supports that.

Setting up GCC build is easy, but for MSVC we have two options

• Run MSVC in Wine, with https://github.com/paleozogt/MSVCDocker
• Run MSVC in Windows Container, see https://devblogs.microsoft.com/cppblog/using-msvc-in-a-docker-container-for-your-c-projects/

The latter is the "official" method, but it will require us to buy a modern Windows Server license (2016, I guess) and set it up somewhere