bugfix: add 'networks' to 'docker-in-docker' (laradock#1660)

I do not know why but the pull request laradock#1655 has lost the 'networks' code. I'll try to insert it Insert again ;-)

Author: vlauciani

docker-compose.yml

@@ -726,6 +726,8 @@ services:
         - ${APP_CODE_PATH_HOST}:${APP_CODE_PATH_CONTAINER}
       expose:
         - 2375
+      networks:
+        - backend
 
 ### NetData ################################################
     netdata: