proxy_cache_background_update after cache expiry

[IgorR]

Hello,

I'm trying to configure nginx to use proxy_cache_background_update but it
seems like after expiry it still waits for the full roundtrip to the
backend, returning a MISS in X-Cache-Status. What am I MISSing?

I'm using nginx 1.11.12 under ubuntu 14.04 running inside docker, but
hopefully this is too much detail.

location ~ ^/?(\d+/[^/]+)?/?$
{
   expires 20s;

   proxy_cache app_cache;
   proxy_cache_lock on;

   proxy_cache_bypass $http_upgrade;

   proxy_pass http://172.17.0.2:5000;
   proxy_http_version 1.1;
   error_log    /nginxerror.log debug;

   add_header X-Cache-Status $upstream_cache_status;

   proxy_cache_use_stale error timeout updating http_500 http_502 http_503
http_504;
   proxy_cache_background_update on;

   break;
}

NB: This is a duplicate of my so question, I was kindly advised on the nginx
IRC to repost here for better chances, the original question is here:
http://stackoverflow.com/questions/43223993/nginx-proxy-cache-background-update-after-cache-expiry

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,273417,273417#msg-273417