Redirect subdomain to port (nginx)

nginx_docker_redirect.conf

# This will redirect a numerical subdomain to a port.
# A use would be Docker contianers.
# This relies on the following DNS record:
# *.test.domain.com IN A 10.100.199.31
# and the fact that this container would run on the same Docker
# host as the backends. 

server {
	listen 80 default_server;
	listen [::]:80 default_server ipv6only=on;

  	# This captures the subdomain (if digits) as the $backport variable
	server_name ~^(?P<backport>[0-9]+)\.test\.domain\.com$;

	location / {
		if ($backport) {
        		resolver 8.8.8.8;
        		proxy_pass http://$server_name:$backport;
		}
	}
}