syncrepo: add X-Served-By header to geo mirrors

Potentially useful to quickly know which mirror we're talking to.

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

@@ -17,6 +17,10 @@ server {
     ssl_certificate_key  /etc/letsencrypt/live/{{ domain }}/privkey.pem;
     ssl_trusted_certificate /etc/letsencrypt/live/{{ domain }}/chain.pem;
 
+{% if 'geo_mirrors' in group_names and domain == geo_mirror_domain %}
+    add_header X-Served-By "{{ inventory_hostname }}";
+{% endif %}
+
     autoindex on;
 }
 {% endfor %}