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.

build-image/docker-entrypoint.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Drops from root (needed to read the prebuilt toolchain under /opt) to a
+# user matching the host's UID/GID, so anything written into the
+# bind-mounted repo (engine/build_ext, engine/.build-output, ...) is owned
+# by the host user instead of root.
+set -euo pipefail
+
+USER_UID="${HOST_UID:-1000}"
+USER_GID="${HOST_GID:-1000}"
+
+groupadd -g "$USER_GID" builder 2>/dev/null || true
+useradd -u "$USER_UID" -g "$USER_GID" -m -s /bin/bash builder 2>/dev/null || true
+
+exec gosu builder "$@"