Add a GitHub action that builds the docker image to run builds for the
Windows (MXE) version of Subsurface.
Also update the MXE image Dockerfile to the latest version of MXE, and
add a patch to use a current version of mdbtools.
Configure GitHub actions that do not build docker images to not trigger
on changes to the contents of `scripts/docker/`.
Signed-off-by: Michael Keller <github@ike.ch>
