Clarify Docker Compose persistence and relative bind mount behavior

[Duckiduc]

Summary

This updates the Docker Compose documentation to make persistence behavior clearer and safer for long-lived deployments.

What changed

• Updated the Docker Compose example in the README to use absolute host path examples
• Documented where Wallos stores persistent data inside the container:
  • /var/www/html/db
  • /var/www/html/images/uploads/logos
• Added an explanation that relative bind mounts are resolved from the Compose project directory on the host
• Clarified that some deployment workflows can change that project directory between redeploys, which may cause Wallos to mount a new empty host path
• Mentioned Portainer as one example of this behavior
• Added a warning in the sample docker-compose.yaml comments to highlight the risk of relative bind mounts in stack-manager or remote deployment setups

Why

Wallos already persists data correctly, but the previous Docker Compose example could be misleading because it used relative bind mounts without explaining their behavior.

When the Compose project directory changes between deployments, Docker can mount a different host directory than before. In that case, Wallos appears to start fresh even though the original data may still exist elsewhere on the host.

Testing

Docs-only change. No runtime tests were required.

Closes #1008