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.

NOTICE.txt

@@ -0,0 +1,9 @@
+This package's original tooling (build scripts, Makefile, launcher) is
+licensed under the MIT License - see LICENSE.
+
+The systemshock binary and its shared libraries in lib/ are built from
+Shockolate (https://github.com/Interrupt/systemshock), which is licensed
+under the GNU GPLv3 - see LICENSE.Shockolate. It is included unchanged
+and is not relicensed by this project's MIT license.
+
+Game assets are not included - see res/GET_ASSETS.txt.