Support new cross-build process, based on new Docker image

The prepared docker image allows cross-building for x86_64 Windows 64, i686 Windows 32, armhf Linux and aaarch64 Linux, plus the native x86_64 and i686 library versions. It may be possible to generate more compact libraries with clang instead of the gcc mingw, however there was no time to test this, and the current solution is tested to be working. This can be addressed at a later time...