[DBBackup] Fix DB networks for Mattermost

pica-db-backup/docker-compose.yml

@@ -13,6 +13,8 @@ networks:
     name: "mobilizon"
   codimd:
     name: "codimd"
+  mattermost-db:
+    name: "mattermost-db"
 
 services:
   db-backup:
@@ -31,4 +33,5 @@ services:
       - wekan
       - mobilizon
       - codimd
+      - mattermost-db
     restart: unless-stopped