fix behavior of nginx when explorer is deactivated

docker/my-dojo/nginx/explorer.conf

@@ -1,9 +1,11 @@
 server {
   listen 9080;
   server_name _;
+  resolver 127.0.0.11 valid=30s;
   
   location / {
-    proxy_pass http://explorer:3002;
+    set $upstream http://explorer:3002;
+    proxy_pass $upstream;
     proxy_http_version 1.1;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection 'upgrade';