Unlike rwgrim/msys2-cross this Dockerfile doesn't require manual setup
as it downloads msys2 itself. Though it uses archlinux as image base
instead of debian in order to reuse archlinux's own pacman.
Loonycyborg already uses these tools with Python 3.
The python launcher tool for Windows has magic handling for some shebang lines,
however `#!/bin/env python` isn't recognised without the `/usr`. Had the `/usr`
been included with the old code then these scripts would likely have been run
with Python 2.
https://docs.python.org/dev/using/windows.html#shebang-lines
Of note:
* The image now created by Dockerfile-base-mingw is available on wesnoth's Docker Hub.
* Strict building is disabled, as there are a couple warnings that show up when crosscompiling.
