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.

.gitignore

@@ -3,16 +3,21 @@
 
 # Game assets are not included in the repo
 /res/assets/setup_system_shock_enhanced*
+/res/assets/*.zip
 /res/assets/ss_ee/
 
 # Build output
 /dist/
+/dist-win/
 /build/
 
 # Engine build artifacts (engine/ is committed as a source snapshot; these
-# are generated by run-image.sh / build-image/build-engine.sh)
+# are generated by run-image.sh / build-image/build-engine.sh /
+# build-image/build-engine-win.sh - the Windows cross-build's own scratch
+# copy of engine/ lives under /build/win-engine/, already covered above)
 /engine/build_ext/
 /engine/.build-output/
+/engine/.build-output-win/
 /engine/CMakeCache.txt
 /engine/CMakeFiles/
 /engine/cmake_install.cmake