Delay DNS resolution from startup

[PeterJCLaw]

If DNS entries for non-local proxied things aren't available we almost certainly want NGINX to still start up. We should check that it has a sensible failure mode if the upstream still isn't around when a request comes in though.

The consensus seems to be that using a variable in the NGINX config is the way to achieve this:

• https://serverfault.com/a/593003
• https://medium.com/@icarobichir/configure-nginx-to-resolve-dns-during-the-application-uptime-ec0c1767e01
  (both say the same thing)

https://www.nginx.com/blog/dns-service-discovery-nginx-plus/ appears to have more detailed information.