site-001.conf

location / {

  if (-f $document_root/system/maintenance.html) {
    return 503;
  }

  try_files $uri $uri/index.html @proxy;
}

error_page 503          @503;
location @503 {
  # Serve static assets if found when in maintenance mode.
  if (-f $request_filename) {
    break;
  }

  # Serve the maintenance page if we're in maintenance mode.
  if (-f $document_root/system/maintenance.html) {
    rewrite ^(.*)$ /system/maintenance.html break;
  }

  # This is a normal 503 so serve that page.
  rewrite ^(.*)$ /503.html break;
}