bug#59515: nginx: Fails to start on boot while upstream service is not yet running

[Jonathan Brielmaier]

When I start my personal server with a radicale service behind a nginx
reverse proxy, nginx fails to start.

The relevant part in the log:
Nov 23 16:02:56 localhost shepherd[1]: Service networking has been started.
Nov 23 16:02:56 localhost shepherd[1]: Service radicale has been started.
Nov 23 16:02:56 localhost shepherd[1]: Service ssh-daemon has been started.
Nov 23 16:02:56 localhost shepherd[1]: [nginx] nginx: [emerg] host not
found in upstream "localhost:5232" in
/gnu/store/y29zl57pprwxbcxfx593s16456kxk99y-nginx.conf:15
Nov 23 16:02:56 localhost shepherd[1]: Failed to start nginx in the
background.

The config can be found here:
https://gitlab.com/jonsger/jonsger-guix/-/blob/master/config/baebia.scm#L116
```
(upstream-blocks (list
  (nginx-upstream-configuration
    (name "radicale")
    (servers (list "localhost:5232")))))
```

I wonder whats going wrong here. Is there a way to define that nginx
service should wait until radicale service is started?

~Jonathan