fix(compose): pre-create pack/custom mountpoint for nested ro bind

The ./custom:/srv/pack/custom mount nests inside the read-only ./pack:/srv/pack
mount. Docker can't mkdir the child mountpoint under a :ro parent, so on a fresh
checkout (no pack/custom/ dir) container init fails with "read-only file system".
Track an empty pack/custom/ so the mountpoint always exists.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -132,6 +132,9 @@ services:
       - ./pack:/srv/pack:ro
       # Custom (locally-hosted) mod jars live OUTSIDE ./pack so packwiz refresh
       # doesn't index them as direct files. Served at pack.${BASE_DOMAIN}/custom/.
+      # Nested under the :ro /srv/pack mount — the mountpoint pack/custom/ must
+      # pre-exist on the host (tracked via pack/custom/.gitkeep) or Docker can't
+      # mkdir it in the read-only parent.
       - ./custom:/srv/pack/custom:ro
       # Static site from ANOTHER repo (absolute host path in .env).
       - ${DISTRIBUTION_WEB_ROOT:?DISTRIBUTION_WEB_ROOT unset in .env}:/srv/distribution:ro