Re: How do I let linux assign the port that nginx listens on?

This can be done by establishing the listening port external to nginX and then using the "NGINX" environment variable to pass the file descriptor to nginX, e.g.:

NGINX="3;" nginx

You need to specify the "listen" clause(s) using the same address as was established for the listening port, e.g.:

http {
server {
listen [::]:53124;
}
}

Notes:

This mechanism is undocumented (quite deliberately, it would appear), but is widely used for systemd-based launches.

The use of the NGINX environment variable implies daemon=off.

One reason you might want to do this sort of thing is to be able to run a number of nginX instances in parallel to perform tests in isolation from each other.