make nginx config specifically use ipv6 localhost loopback

docs/rest_api/nginx

@@ -7,7 +7,7 @@ server {
     ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
 
     location / {
-        proxy_pass http://localhost:5001;
+        proxy_pass http://[::1]:5001;
         proxy_set_header Host $host:$port;
         proxy_set_header X-Forwarded-For $remote_addr;
     }