Set custom Cache-Control header for wp-supercache

Drops the 'must-revalidate' bit, which prevents serving stale pages,
and sets a longer max-age to match the edge NGINX caching ttl.

docker/wp-config.php

@@ -115,6 +115,10 @@ define('SMTP_DEBUG', 0);
  */
 define('COOKIEHASH', '7d12c307721c1cbe9412c639209d72aa');
 
+/* Custom Cache-Control headers for WP-SuperCache.
+ * These are set to match our edge nginx caches.
+define('WPSC_CACHE_CONTROL_HEADER', 'max-age=600');
+
 /**#@+
  * Authentication Unique Keys and Salts.
  *