Set X-Real-IP header for rebuilderd

Rebuilderd since 0.6.0 supports the X-Real-IP header to show what
builder is working on a task.

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

@@ -46,6 +46,7 @@ server {
     }
 
     location /api {
+            proxy_set_header    X-Real-IP          $remote_addr;
 	    proxy_pass http://127.0.0.1:8484;
     }