I don’t think there is a “TrueNAS supported” way of making it work. But you can always add the user to the docker group manually:
sudo gpasswd -a "$USER" docker
Note: This only affects new ssh sessions. After that you can execute docker without sudo. Might break after a reboot and/or update, but can probably be added to a init script if really required.
Edit: For completness, to remove the user from the docker group:
sudo gpasswd -d "$USER" docker