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>
