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.

run-image.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Runs the build-image against the repo to compile engine/ (the vendored
+# Shockolate snapshot), entirely offline - every dependency it needs
+# (SDL2, SDL2_mixer, fluidsynth-lite, a MIDI soundfont) was already built
+# into the image by ./build-image.sh.
+#
+# Output lands in engine/.build-output/ - run `make dist` afterwards to
+# assemble it (together with the game assets from res/assets/ss_ee/) into
+# dist/.
+#
+# Usage: ./run-image.sh [command...]
+#   With no arguments, runs the engine build. Pass a command (e.g. `bash`)
+#   to get a shell in the build environment instead, useful for debugging
+#   a failed build.
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+ROOT="$(pwd)"
+
+fail() { echo "PREFLIGHT FAIL: $*" >&2; exit 1; }
+
+command -v docker >/dev/null 2>&1 \
+    || fail "docker not found in PATH"
+
+IMAGE="${REGISTRY_IMAGE:-questshock/builder}"
+if [ -f "$ROOT/registry.env" ]; then
+    # shellcheck disable=SC1091
+    source "$ROOT/registry.env"
+    IMAGE="${REGISTRY_IMAGE:-$IMAGE}"
+fi
+
+IMAGE_TAG="$(<"$ROOT/build-image/VERSION")"
+[ -n "$IMAGE_TAG" ] || fail "build-image/VERSION is empty"
+IMAGE="$IMAGE:$IMAGE_TAG"
+
+docker image inspect "$IMAGE" >/dev/null 2>&1 \
+    || fail "$IMAGE not found locally - run ./build-image.sh first"
+
+TTY_FLAGS="-i"
+[ -t 1 ] && TTY_FLAGS="-it"
+
+# shellcheck disable=SC2086
+docker run --rm $TTY_FLAGS \
+    -v "$ROOT:/workspace" \
+    -e HOST_UID="$(id -u)" \
+    -e HOST_GID="$(id -g)" \
+    "$IMAGE" \
+    "$@"
+
+if [ "$#" -eq 0 ]; then
+    echo
+    echo "This only compiled the engine (engine/.build-output/) - it does not"
+    echo "create dist/ by itself. Run 'make dist' next to assemble a runnable"
+    echo "copy (or 'make package' for a redistributable tarball)."
+fi