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.

registry.env.example

@@ -0,0 +1,9 @@
+# Copy to registry.env (gitignored, never commit the real values) and fill
+# in your registry credentials. Used by build-image.sh, run-image.sh, and
+# upload-image.sh. Only REGISTRY_IMAGE is needed for local builds/runs;
+# REGISTRY/REGISTRY_USER/REGISTRY_PASSWORD are only needed to push with
+# upload-image.sh.
+REGISTRY=cr.ladkau.de
+REGISTRY_IMAGE=cr.ladkau.de/questshock/builder
+REGISTRY_USER=
+REGISTRY_PASSWORD=