docker-compose does not recursively expand nested `${...}` in `:-`
defaults, so `TURN_DOMAIN=${TURN_DOMAIN:-${LIVEKIT_DOMAIN}}` leaked the
literal string `${LIVEKIT_DOMAIN}` into the container, breaking TURN
and WebRTC connectivity. Set the default in .env.sample where abra
shell-expands it before docker-compose sees it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
