archwiki: Improve cache hit ratio by using background updating and cache locking

This should not cause any issues as MediaWiki should purge stale content
from the cache (by sending a PURGE request to the nginx-cache-purge
service).

roles/archwiki/templates/nginx.d.conf.j2

@@ -121,6 +121,9 @@ server {
         # [1] https://www.fastly.com/blog/best-practices-using-vary-header/
         # [2] https://github.com/nginx/nginx/commit/1332e76b20a6a1e871904525d42b17dcaed81eec
         fastcgi_ignore_headers Vary;
+        fastcgi_cache_background_update on;
+        fastcgi_cache_use_stale updating;
+        fastcgi_cache_lock on;
 
         add_header X-Cache $upstream_cache_status;
 {% endblock %}