Add Gunicorn to nginx_shell.nix

[toraritte]

There is a just recipe for Gunicorn, but NGINX operationally depends on it, so it makes sense to start Gunicorn before NGINX is fired up.

The current method in dev is:
0. nix-shell ... dev_shell.nix

1. just gunicorn_local 8000 &
2. nix-shell ... nginx_shell.nix

The problem with this is that shells are being stacked (a lot) and when tearing them down, gunicorn never stops on its own.

This would work in a shell.nix too, but clean-up would be painful:

• Gunicorn does save its PID in a pidfile, but the problem is that there can be multiple one present, so which one is the actual one? just gunicorn assigns a name dynamically based on a timestamp.

• jobs -p shows the PID of the jobs, but would this work in a shell.nix as well? (Caveat: none of the PIDs in the pidfiles will match the output of jobs -p because Gunicorn is started using an intermediary, just, therefore it will have a parent.)

Options:

• Use systemd.

  No PID-lookup shenanigans, only a single command - but what if multiple instances needs to be fired up? (For whatever reason.)

• Call Gunicorn directly, without just.

  This way, the reference to the pidfile can be retained and directly used in the clean-up phase. (I don't think that it needs to be save in a shell variable, because the after constructing the pidfile's name, every reference to it in the shell.nix will work the same as being in a closure.