With help from another thread, I was able to run this commnand:
TrueNAS% sudo docker exec -it -u 33 ix-nextcloud-nextcloud-1 php -f /var/www/html/occ user:resetpassword Geoffrey
That let me reset the password, but I still can’t log in - this may not be a Password issue, but a trust / security issue in the config of Nextcloud?