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>

scripts/docker/mxe-build-container/Dockerfile

@@ -0,0 +1,74 @@
+# Build the image using the --build-arg option, e.g.:
+# docker build -t boret/myimage:0.1 --build-arg=mxe_sha=123ABC456 .
+#
+
+# Start from Ubuntu
+From ubuntu:20.04
+
+# very often master is broken, so we pass in a known good SHA
+ARG mxe_sha=master
+ENV _ver=${mxe_sha}
+
+# update and set up the packages we need for this cross build
+RUN apt-get update  &&  apt-get upgrade -y && \
+DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
+    autoconf \
+    automake \
+    autopoint \
+    bash \
+    binutils \
+    bison \
+    bzip2 \
+    ca-certificates \
+    flex \
+    g++ \
+    g++-multilib \
+    gettext \
+    git \
+    gperf \
+    intltool \
+    libc6-dev-i386 \
+    libgdk-pixbuf2.0-dev \
+    libltdl-dev \
+    libssl-dev \
+    libtool \
+    libtool-bin \
+    libxml-parser-perl \
+    make \
+    openssl \
+    p7zip-full \
+    patch \
+    perl \
+    pkg-config \
+    python \
+    ruby \
+    sed \
+    unzip \
+    wget \
+    xz-utils \
+    lzip \
+    scons
+
+# checkout MXE at the right version
+RUN mkdir -p /win
+RUN cd /win ; git clone git://github.com/mxe/mxe ; \
+    cd mxe ; \
+    git checkout ${_ver} ;
+
+# Patch the qtconnectivity build to explicilty enable native-win32-bluetooth and ensure another
+# backend is not picked
+ADD qtconnectivity-1.patch /win/mxe/src/qtconnectivity-1.patch
+
+# Move the settings into place to build everything that we need
+# separate download from build so that we can redo the build
+ADD settings.mk /win/mxe/settings.mk
+RUN cd /win/mxe ; \
+    make -j 6 download 2>&1 | tee mxe-build.log
+RUN cd /win/mxe ; \
+    make -j 6 2>&1 | tee -a mxe-build.log ;
+
+# for some reason smtk2ssrf needs a static build of mdbtools
+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/*