Aldo,
The Docker monitoring that you're using is monitoring the Docker platform, and therefore can only see metrics at that level. Filesystem utilization is a concept that is specific to the operating systems of the individual containers, so you can only get it by running commands in the container.
The good news is that Docker does provide a way to run commands in the container: docker exec.
You could use that to write a script that runs 'df' or something on each running container.
Regards,
Tim