Update firststeps-rp.md

prevent certdumper from creating his own network and messing around with the backup/restore-script. Certdumper doesn't need network at all

docs/firststeps-rp.md

@@ -196,6 +196,7 @@ services:
     certdumper:
         image: humenius/traefik-certs-dumper
         container_name: traefik_certdumper
+        network_mode: none
         volumes:
           # mount the folder which contains Traefik's `acme.json' file
           #   in this case Traefik is started from its own docker-compose in ../traefik