Watchtower container has Docker API mismatch (client 1.25 vs daemon 1.54+)
that can't be worked around without upstream fix. Simple cron job does the
same thing more reliably:
- Every 5 min: docker compose pull + detect digest change + up -d if changed
- Logs to /var/log/agnes-auto-upgrade.log
This removes the watchtower container and a Docker daemon dependency.