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.

upload-image.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Pushes the build-image - already built locally with ./build-image.sh,
+# and ideally verified with ./run-image.sh - to a container registry, so
+# CI or other machines can reuse it without rebuilding the whole toolchain.
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+ROOT="$(pwd)"
+
+fail() { echo "PREFLIGHT FAIL: $*" >&2; exit 1; }
+
+command -v docker >/dev/null 2>&1 \
+    || fail "docker not found in PATH"
+
+[ -f "$ROOT/registry.env" ] \
+    || fail "registry.env not found - copy registry.env.example to registry.env and fill in your registry credentials"
+
+# shellcheck disable=SC1091
+source "$ROOT/registry.env"
+
+: "${REGISTRY:?registry.env must set REGISTRY}"
+: "${REGISTRY_IMAGE:?registry.env must set REGISTRY_IMAGE}"
+: "${REGISTRY_USER:?registry.env must set REGISTRY_USER}"
+: "${REGISTRY_PASSWORD:?registry.env must set REGISTRY_PASSWORD}"
+
+VERSION="$(<"$ROOT/build-image/VERSION")"
+[ -n "$VERSION" ] || fail "build-image/VERSION is empty"
+
+docker image inspect "$REGISTRY_IMAGE:$VERSION" >/dev/null 2>&1 \
+    || fail "$REGISTRY_IMAGE:$VERSION not found locally - run ./build-image.sh first"
+
+echo "== Logging in to $REGISTRY =="
+echo "$REGISTRY_PASSWORD" | docker login "$REGISTRY" -u "$REGISTRY_USER" --password-stdin
+
+echo "== Pushing $REGISTRY_IMAGE:$VERSION and :latest =="
+docker push "$REGISTRY_IMAGE:$VERSION"
+docker push "$REGISTRY_IMAGE:latest"
+
+echo "== Done =="
+echo "Image: $REGISTRY_IMAGE:$VERSION"