build-system: create an MXE docker container for both 32 & 64 bit

This should allow us to then do both 32 and 64 bit Windows builds in our CI/CD
and of course for our releases.

In order to still be able to use this container in a GitHub action, aggressively
remove things that we won't need during the build. Since we use the experimental
-squash argument during docker build, this should get us a much smaller container
image in the end.

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

scripts/docker/mxe-build-container/Dockerfile

@@ -72,3 +72,4 @@ RUN cd /win/mxe ; \
     make MXE_TARGETS=x86_64-w64-mingw32.static glib mdbtools -j 6 2>&1 | tee -a mxe-build.log ;
 
 RUN apt-get remove -y bison flex gperf libc6-dev-i386 libgdk-pixbuf2.0-dev libxml-parser-perl python ruby xz-utils scons && rm -rf /var/lib/apt/lists/*
+RUN rm -rf /win/mxe/pkg /win/mxe/log /win/mxe/docs

scripts/docker/mxe-build-container/settings.mk

@@ -6,7 +6,7 @@
 JOBS := 8
 
 # This variable controls the targets that will build.
-MXE_TARGETS :=  x86_64-w64-mingw32.shared
+MXE_TARGETS :=  x86_64-w64-mingw32.shared i686-w64-mingw32.shared
 
 # The three lines below makes `make` build these "local packages" instead of all packages.
 # The ordering of the list appears weird, but this seems to help to get the build done