[add] add custom error page if upstream fails

proxy/files/nginx.conf

@@ -18,6 +18,18 @@ http {
     # set larger name bucket for large domain names
     server_names_hash_bucket_size  64;
     
+    map $status $status_category {
+        "~^(?<code>(?<prefix>(?<category>[1-5])[0-9])[0-9])$" "$category";
+    }
+
+    map $status $status_prefix {
+        "~^(?<code>(?<prefix>(?<category>[1-5])[0-9])[0-9])$" "$prefix";
+    }
+
+    map $server_name $server_domain {
+        "~(?<domain_name>[a-zA-Z0-9-]*)(\.[a-z]*)$" "$domain_name";
+    }
+    
     server {
         listen 8080 default_server;