Vendor the Shockolate engine and build it via a Docker image with every

dependency (SDL2, SDL2_mixer, fluidsynth-lite, a MIDI soundfont)
prebuilt, so compiling the engine needs no network access - just the
image and the engine source. Add res/assets/extract_assets.sh to pull
the game's data files out of a purchased GOG installer, and a Makefile
that assembles a runnable dist/ from the two.

Also add `make package`, which builds a redistributable tarball that
omits the proprietary game assets (shipping res/GET_ASSETS.txt instead)
plus license information for both the MIT tooling and the GPLv3 engine,
and a Gitea Actions workflow that builds and publishes it to
dl.ladkau.de on every push.

engine/osx-linux/readme_osx_linux.md

@@ -0,0 +1,6 @@
+Shockolate - Portable System Shock
+============
+
+1. Copy `res/` folder of original System Shock to this directory
+2. Install SDL2, SDL2_mixer and Fluidsynth by running `./install_<bits>bit_sdl.sh` where `<bits>` is 32 or 64, depending on your system capabilities. Linux needs `sudo` for this script. 
+3. Run Shockolate `./run.sh`