nginx proxy won't retry if backend is unavailable

Question

I have an application that sits behind an nginx reverse proxy. Nothing fancy, just a simple

server {
  listen 54321;

  proxy_connect_timeout 5000ms;
  proxy_read_timeout 5000ms;

  location / {
    proxy_pass http://localhost:54322/;
  }
}

nginx starts first. If I try to hit the application while it is down, I get a 502 Bad Gateway. That's expected but if I start the application, I still get a 502. It is only after I restarted nginx that requests finally go through.

How can I instruct nginx to keep trying (at least periodically) to connect to the backend?