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>
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