Use docker links for nginx to resolve properly

nginx/nginx.conf

@@ -33,15 +33,11 @@ http {
     	}
 
       location / {
-        resolver 127.0.0.11;
-        set $webmail_backend "http://webmail";
-        proxy_pass $webmail_backend;
+        proxy_pass http://webmail;
       }
 
       location /admin {
-        resolver 127.0.0.11;
-        set $admin_backend "http://admin";
-        proxy_pass $admin_backend;
+        proxy_pass http://admin;
       }
     }
 }