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.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Builds the engine build-image (build-image/Dockerfile) locally, tagged
+# with build-image/VERSION and :latest. Does not push - test it with
+# ./run-image.sh first, then publish with ./upload-image.sh.
+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
+
+VERSION="$(<"$ROOT/build-image/VERSION")"
+[ -n "$VERSION" ] || fail "build-image/VERSION is empty"
+
+echo "== Building $IMAGE:$VERSION =="
+docker build \
+    -f "$ROOT/build-image/Dockerfile" \
+    -t "$IMAGE:$VERSION" \
+    -t "$IMAGE:latest" \
+    "$ROOT"
+
+echo "== Done =="
+echo "Image: $IMAGE:$VERSION (and :latest)"
+echo "Test it with ./run-image.sh, then publish with ./upload-image.sh"