Pre-create /data/passthrough in the image so non-root startup works
Regression from the security-hardening commit: switching to USER
spacebee (uid 10001) meant Passthrough.__init__'s implicit `/data`
parent mkdir could no longer succeed — UID 10001 has no write access
to /. Pre-create /data/passthrough at build time and chown to the
runtime user so the container boots with or without a mounted volume.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>