I found a solution to mounting a NAS share on HassOS system. My system is running HassOS 2021.4.6
I could not get shell_command to work for using mount. I tried run a shell script from shell_command with mount in it but the mount part always failed.
Using the SSH & Web Terminal add-on from Community add-ons the mount command worked as did the shell script. So I expect the shell_command and the terminal are in different containers as discussed in posts above. (I’m not great on the underlying architectures)
So, following an example in the documentation for the add-on, I created an Automation triggered on HA Start.
alias: Mount Images at boot description: Mount the Reolink folder at boot. trigger: - platform: homeassistant event: start condition:  action: - delay: hours: 0 minutes: 1 seconds: 0 milliseconds: 0 - service: hassio.addon_stdin data: addon: a0d7b954_ssh input: >- mount -t nfs4 10.1.1.16:/volume1/surveillance/Reolink /config/www/images/Reolink mode: single
In this example I am mounting an NFS share but CIFS also works.
I spent the weekend trying to get the shell_command working without success. This worked first try