[Fix] Mailhog should be exposed to backend

- fixes failing connection to host "mailhog" when sending mail via smtp
- mailhog needs to be available for the backend at Port 1025
- i.e. in a Laravel app .env should contain "MAIL_HOST=mailhog

docker-compose.yml

@@ -518,6 +518,7 @@ services:
         - "8025:8025"
       networks:
         - frontend
+        - backend
 
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