Don't run TrueNAS as docker, first off.
Second, you can find the environment variable to change this behavior (if it even exists), or you have the option of changing whatever you're pulling as the source, changing the filesystem, committing that, then changing the target image to be the resulting image.
Edit: read this chain so I don't have to be the asshole here: https://www.truenas.com/community/threads/nginx-pod-and-configuration.116006/