Snap-installed Docker does not automatically create a docker group,
causing permission denied errors when the pulse-docker service user
tries to access /var/run/docker.sock.
Changes:
- Auto-detect Snap Docker installations
- Create docker group if missing when Snap Docker is detected
- Restart Snap Docker after group creation to refresh socket ACLs
- Add socket access validation before starting the service
- Handle symlinked Docker sockets in systemd unit ReadWritePaths
- Document troubleshooting steps in DOCKER_MONITORING.md
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