Add dockerfile for making mingw cross-compile builds

utils/dockerbuilds/make_mingw_build

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+#!/bin/sh -xe
+
+cd mingw
+docker build -t mingw-wesnoth .
+docker run -it -v "$PWD"/../../..:/wesnoth -v "$PWD"/../mingwbuild:/output mingw-wesnoth

utils/dockerbuilds/mingw/Dockerfile

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+FROM rwgrim/msys2-cross
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && \
+    apt-get install -y scons g++-mingw-w64-x86-64 pkg-config python3-pefile && \
+    update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists
+
+RUN pacman-cross -S --noconfirm \
+    mingw-w64-x86_64-boost \
+    mingw-w64-x86_64-SDL2 \
+    mingw-w64-x86_64-SDL2_image \
+    mingw-w64-x86_64-SDL2_mixer \
+    mingw-w64-x86_64-SDL2_ttf \
+    mingw-w64-x86_64-pango
+
+COPY get_dlls.py /scripts/get_dlls.py
+
+ENTRYPOINT mkdir /build && cd /build && scons -j `nproc` arch=x86-64 prefix=/windows/mingw64 gtkdir=/windows/mingw64 host=x86_64-w64-mingw32 -Y /wesnoth && cp /build/wesnoth.exe /output/ && cd /output && python3 /scripts/get_dlls.py

utils/dockerbuilds/mingw/get_dlls.py

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+#!/bin/env python
+
+import pefile, pathlib, shutil
+
+dlls = set()
+dllpath = pathlib.Path('/windows/mingw64/bin')
+pe_modules = set([pefile.PE('wesnoth.exe')])
+
+while pe_modules:
+    pe = pe_modules.pop()
+    for entry in pe.DIRECTORY_ENTRY_IMPORT:
+        path = dllpath / pathlib.Path(entry.dll.decode())
+        if path not in dlls and path.exists():
+            dlls.add(path)
+            pe_modules.add(pefile.PE(path))
+
+for dll in dlls:
+    shutil.copy(dll, ".")