Cite (commented) "dns" in docker-compose.yml

When a user use opencloud under a reverse proxy for a domain not exposed to public internet is important to specify the IP address of his Reverse Proxy service.
This simple addition to `docker-compose.yml` will save minutes/hour of Google/AI searches!


```
dns:
  - x.y.z.w
dns_search:
  - custom.domain.local
```