build-system: Docker build for 64bit MXE

We previously tried to build the MXE Docker container on GitHub using
an Action, but that really didn't work well and was a lot more trouble
than it was worth.

So this goes back to an offline build mechanism where I simply create
an updated Docker image when needed and push that to Docker Hub.

But this nearly hides the most interesting change here - we are finally
switching to using 64bit binaries on Windows. It's 2020 and fewer than
1% of our users use 32bit Windows machines. We'll need to expand this
to be able to have both a 32bit and a 64bit version of Subsurface for
Windows. But for now, this solves the problem for 99% of our users.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

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