SIGTERM is the default signal sent to containers to stop them: https://docs.docker.com/engine/reference/commandline/stop/
STOPSIGNAL does allow you to override the default signal sent to the container. Leaving it out of the Dockerfile causes no harm – it will remain the default of SIGTERM.
This being said, it is unclear why the author has explicitly defined the STOPSIGNAL as SIGTERM.
Looking at this commit, we can see that the STOPSIGNAL used to be set to SIGQUIT.
My guess is that they left it in explicitly for documentation’s sake after making the change.
Discussion of the change here: https://github.com/nginxinc/docker-nginx/issues/167