Add Windows cross-compilation via MinGW, and fix Docker build permission

issues on vboxsf-mounted checkouts

New `make dist-win`/`package-win` targets (folded into `dist`/`package`
alongside the renamed `dist-linux`/`package-linux`) cross-compile
systemshock.exe via MinGW, using prebuilt SDL2/SDL2_mixer/GLEW/
fluidsynth-lite baked into the build-image - no Windows machine or Wine
needed to build it, confirmed working and playable on a real Windows
machine. dist-win/ ships DLLs flat alongside the exe plus a new
res/run.bat launcher, packaged into a .zip the same way dist/ becomes a
.tar.gz.

Also fixes three build-image bugs hit while testing on a VirtualBox
vboxsf-mounted checkout: build-engine.sh/docker-entrypoint.sh losing
their execute bit (chmod +x on restrictive source perms), the container
user missing access to /workspace's supplementary vboxsf group, and
cp -a failing on symlink/hard-link creation (vboxsf doesn't support
either) - now falls back to dereferencing copies when detected. Also
adds Docker/zip/etc. prerequisites to the README for both the desktop
and Quest builds.

build-image/Dockerfile

@@ -18,6 +18,13 @@ ARG FLUIDSYNTH_LITE_REF=c539a8d9270ba5a3f7d6e460606483fc2ab1eb61
 # Soundfont used for MIDI music, matching what engine/build_deps.sh itself
 # fetches (a free substitute for the Windows default GM soundfont).
 ARG SOUNDFONT_URL=http://rancid.kapsi.fi/windows.sf2
+# GLEW, for the Windows/MinGW cross-build only (engine/CMakeLists.txt's
+# WIN32 branch - Windows' own opengl32.dll only exposes OpenGL 1.1, so
+# anything newer needs GLEW's runtime extension loading; Linux instead
+# gets modern prototypes straight from Mesa's headers, no loader needed -
+# see engine/src/MacSrc/OpenGL.cc). Matches the version engine/'s own
+# upstream Windows build script (build_win64.sh) used.
+ARG GLEW_VERSION=2.1.0
 # Gitea/GitHub Actions' JS-based actions (actions/checkout,
 # actions/upload-artifact, ...) need a node binary in the container job's
 # PATH - this image is otherwise pure C toolchain, so it isn't pulled in
@@ -99,6 +106,15 @@ ENV DEBIAN_FRONTEND=noninteractive
 # openssh-client: the CI workflow's `sftp` publish step.
 # openjdk-17-jdk-headless: Gradle/AGP's own minimum JDK for the APK build.
 # unzip: extracts the Android cmdline-tools zip below.
+# mingw-w64: the x86_64-w64-mingw32-{gcc,g++,windres,ar,...} cross
+# toolchain for the Windows desktop build (see the "Windows cross-compile"
+# section below). Ubuntu ships both a win32-thread-model and a
+# posix-thread-model variant behind update-alternatives; the default
+# (win32) is fine here since nothing in engine/ uses std::thread.
+# zip: `make package-win`'s Windows release archive - a CI job's own
+# `make package` runs inside this image as its container (see
+# .gitea/workflows/build.yml), so it needs to be baked in here, not just
+# available on a local dev machine's own host (see README).
 #
 # All apt installs deliberately live in this one RUN, first, so editing
 # anything below it (in particular the Android cross-compile steps, the
@@ -111,7 +127,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         libgl1-mesa-dev libglx-dev libxext-dev libx11-dev libxrandr-dev \
         libxi-dev libxfixes-dev libxss-dev libxinerama-dev libxcursor-dev \
         libogg-dev libvorbis-dev libasound2-dev openssh-client \
-        openjdk-17-jdk-headless unzip \
+        openjdk-17-jdk-headless unzip mingw-w64 zip \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /opt/prebuilt
@@ -150,10 +166,96 @@ RUN git clone https://github.com/EtherTyper/fluidsynth-lite.git \
     && rm -rf .git
 
 # General MIDI soundfont for fluidsynth playback - engine/build_deps.sh
-# fetches the same file and drops it into engine/res/.
+# fetches the same file and drops it into engine/res/. Shared by both the
+# Linux and Windows builds below.
 RUN mkdir -p soundfont \
     && curl -sSL -o soundfont/default.sf2 "${SOUNDFONT_URL}"
 
+# CMake toolchain file for the Windows/MinGW cross-build below, reused at
+# container-run time by build-image/build-engine-win.sh (see
+# MINGW_TOOLCHAIN_FILE) to cross-compile engine/ itself the same way. GCC's
+# runtime (libgcc/libstdc++) is linked statically so only the SDL2/
+# SDL2_mixer/GLEW/fluidsynth-lite DLLs (plus libwinpthread, which isn't
+# safe to static-link the same way) need shipping alongside systemshock.exe.
+ENV MINGW_TOOLCHAIN_FILE=/opt/mingw-toolchain.cmake
+RUN printf '%s\n' \
+        'set(CMAKE_SYSTEM_NAME Windows)' \
+        'set(CMAKE_SYSTEM_PROCESSOR x86_64)' \
+        'set(CMAKE_C_COMPILER x86_64-w64-mingw32-gcc)' \
+        'set(CMAKE_CXX_COMPILER x86_64-w64-mingw32-g++)' \
+        'set(CMAKE_RC_COMPILER x86_64-w64-mingw32-windres)' \
+        'set(CMAKE_FIND_ROOT_PATH /usr/x86_64-w64-mingw32)' \
+        '# LIBRARY/INCLUDE/PACKAGE deliberately left at CMake'"'"'s own' \
+        '# cross-compiling default (BOTH) - engine/CMakeLists.txt'"'"'s' \
+        '# BUNDLED SDL2/SDL2_mixer/FluidSynth find_library() calls point at' \
+        '# build_ext/ (outside this sysroot entirely), which ONLY would' \
+        '# refuse to search.' \
+        'set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)' \
+        'set(CMAKE_EXE_LINKER_FLAGS_INIT "-static-libgcc -static-libstdc++")' \
+        > "${MINGW_TOOLCHAIN_FILE}"
+
+# SDL2 and SDL2_mixer for Windows: unlike the desktop build above, these
+# are libsdl.org's own official prebuilt MinGW devel packages (headers +
+# import libs + DLLs for x86_64-w64-mingw32), not built from source here -
+# SDL2's autotools setup targets *nix; upstream ships MinGW builds
+# pre-made instead, same as engine/'s own upstream Windows build script
+# (build_win64.sh) uses. Same version pins as the Linux build above, so
+# both desktop builds ship the same SDL2/SDL2_mixer release.
+RUN curl -sSLO "https://www.libsdl.org/release/SDL2-devel-${SDL2_VERSION}-mingw.tar.gz" \
+    && tar xf "SDL2-devel-${SDL2_VERSION}-mingw.tar.gz" \
+    && mkdir -p /opt/prebuilt/win \
+    && mv "SDL2-${SDL2_VERSION}/x86_64-w64-mingw32" /opt/prebuilt/win/sdl2 \
+    && rm -rf "SDL2-${SDL2_VERSION}" "SDL2-devel-${SDL2_VERSION}-mingw.tar.gz"
+
+RUN curl -sSLO "https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-devel-${SDL2_MIXER_VERSION}-mingw.tar.gz" \
+    && tar xf "SDL2_mixer-devel-${SDL2_MIXER_VERSION}-mingw.tar.gz" --exclude=Xcode \
+    && mv "SDL2_mixer-${SDL2_MIXER_VERSION}/x86_64-w64-mingw32" /opt/prebuilt/win/sdl2_mixer \
+    && rm -rf "SDL2_mixer-${SDL2_MIXER_VERSION}" "SDL2_mixer-devel-${SDL2_MIXER_VERSION}-mingw.tar.gz"
+
+# GLEW for Windows: compiled directly instead of via GLEW's own bundled
+# cross-compile Makefile configs (config/Makefile.linux-mingw64 et al) -
+# those pass raw `-soname`/`--out-implib` straight to whatever $(LD) is
+# set to, which only works if LD is the real `ld` binary, not gcc-as-
+# linker-driver (and that config also hardcodes a 32-bit `i686-w64-
+# mingw32` host despite the "64" in its name) - simpler and more robust
+# to just compile+link GLEW's one source file ourselves. Produces
+# glew32.dll (to ship alongside systemshock.exe) and libglew32.dll.a (the
+# MinGW import library engine/CMakeLists.txt's WIN32 branch links
+# against).
+RUN curl -sSL -o "glew-${GLEW_VERSION}.tgz" \
+        "https://sourceforge.net/projects/glew/files/glew/${GLEW_VERSION}/glew-${GLEW_VERSION}.tgz/download" \
+    && tar xf "glew-${GLEW_VERSION}.tgz" \
+    && cd "glew-${GLEW_VERSION}" \
+    && mkdir -p /opt/prebuilt/win/glew/include/GL /opt/prebuilt/win/glew/lib \
+    && x86_64-w64-mingw32-gcc -DGLEW_NO_GLU -O2 -Iinclude -c src/glew.c -o glew.o \
+    && x86_64-w64-mingw32-gcc -shared \
+        -Wl,--out-implib,/opt/prebuilt/win/glew/lib/libglew32.dll.a \
+        -o /opt/prebuilt/win/glew/lib/glew32.dll \
+        glew.o -lopengl32 -lgdi32 -luser32 -lkernel32 \
+    && cp include/GL/glew.h include/GL/wglew.h /opt/prebuilt/win/glew/include/GL/ \
+    && cd .. && rm -rf "glew-${GLEW_VERSION}" "glew-${GLEW_VERSION}.tgz"
+
+# fluidsynth-lite for Windows: same source/ref/DLL-mode patch as the
+# desktop build above, cross-compiled via the MinGW toolchain file. WIN32
+# skips fluidsynth-lite's own pthread dependency (see its CMakeLists.txt),
+# so no libwinpthread linkage to worry about here.
+RUN git clone https://github.com/EtherTyper/fluidsynth-lite.git fluidsynth-lite-win \
+    && cd fluidsynth-lite-win \
+    && git checkout "${FLUIDSYNTH_LITE_REF}" \
+    && sed -i 's/DLL"\ off/DLL"\ on/' CMakeLists.txt \
+    && rm -rf .git \
+    && cd .. \
+    && cmake -S fluidsynth-lite-win -B build-fluidsynth-win \
+        -DCMAKE_TOOLCHAIN_FILE="${MINGW_TOOLCHAIN_FILE}" \
+    && cmake --build build-fluidsynth-win -j"$(nproc)" \
+    && mkdir -p /opt/prebuilt/win/fluidsynth-lite/lib /opt/prebuilt/win/fluidsynth-lite/include \
+    && cp build-fluidsynth-win/src/*.dll build-fluidsynth-win/src/*.dll.a \
+        /opt/prebuilt/win/fluidsynth-lite/lib/ \
+    && cp -a fluidsynth-lite-win/include/. /opt/prebuilt/win/fluidsynth-lite/include/ \
+    && cp build-fluidsynth-win/include/fluidsynth/version.h \
+        /opt/prebuilt/win/fluidsynth-lite/include/fluidsynth/version.h \
+    && rm -rf fluidsynth-lite-win build-fluidsynth-win
+
 # Node.js: needed only so Gitea/GitHub Actions' JS-based actions can run
 # when this image is used as a CI job's container - see NODE_VERSION above.
 RUN curl -sSL -o /tmp/node.tar.xz \
@@ -283,7 +385,15 @@ RUN git clone --branch "release-${ANDROID_OPENXR_VERSION}" --depth 1 \
 
 COPY build-image/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
 COPY build-image/build-engine.sh /usr/local/bin/build-engine.sh
-RUN chmod +x /usr/local/bin/docker-entrypoint.sh /usr/local/bin/build-engine.sh
+# 755, not +x: the entrypoint drops to a `builder` user matching the
+# *host's* UID/GID (see docker-entrypoint.sh), which is never in these
+# root-owned files' group, so it needs the image's own explicit
+# world-read+execute here - `chmod +x` alone only adds execute bits on
+# top of whatever "other" permissions the source file happened to have
+# (which depends on the host's umask at checkout, e.g. a restrictive
+# 0007 umask yields unreadable-by-other files, which then round-trip
+# into the image and cause a Permission denied at container run time).
+RUN chmod 755 /usr/local/bin/docker-entrypoint.sh /usr/local/bin/build-engine.sh
 
 # Marks a shell as already running inside this image (with every engine
 # build dependency prebuilt above) - lets the Makefile's `engine` target