Do not show error page for 503

WP maintenance mode plugin will return 503

src/nginxconfig.ejs

@@ -108,7 +108,9 @@ server {
 <% } -%>
 
     proxy_http_version 1.1;
+    # intercept errors (>= 400) and use the error_page handler
     proxy_intercept_errors on;
+    # nginx will return 504 on connect/timeout errors
     proxy_read_timeout       3500;
     proxy_connect_timeout    3250;
 
@@ -125,7 +127,8 @@ server {
 
     # only serve up the status page if we get proxy gateway errors
     root <%= sourceDir %>/dashboard/dist;
-    error_page 502 503 504 /appstatus.html;
+    # some apps use 503 to indicate updating or maintenance
+    error_page 502 504 /appstatus.html;
     location /appstatus.html {
         internal;
     }