Issue "sudo docker ps -a" and identify your docker. Under NAMES - this will tell you the name of the Zoneminder docker.
Use this name in the next command:
sudo docker exec -it <docker-name> bash
This will log you into the docker container and you can then edit the /etc/fstab file.
Once done - exit the docker container and issue a "sudo docker restart <docker-name>"
A better alternative is to review the script that you use to launch the Zoneminder docker; there should be a parameter there that controls the /dev/shm size.