What does it mean to to “mount a volume” below?
However, this will run without any configuration what-so-ever, so you'll want to mount a volume with your configurations, a sample configuration is provided in this clone. So if your current working directory is this clone, you could mount the example configurations in /etc/asterisk however, I recommend you create your own configurations.
Docker containers run inside Linux. Firstly, what is there to mount? The filesystem is already mounted. Secondly, why? The container, the OS in the container, is already running? What would be achieved by mounting anything?