Fixing can't login to PHPMyAdmin

fix laradock#754, login to PHPMyAdmin get "laradock#2005 - Unknown MySQL server host 'mysql' (-2)" error.

Author: bestlong

docker-compose.yml

@@ -392,6 +392,7 @@ services:
         - "${PMA_DB_ENGINE}"
       networks:
         - frontend
+        - backend
 
 ### Adminer Container ####################################