I am not using the docker AIO NC but you should try to use occ commands to set system config settings by using a command like this
sudo docker exec --user www-data -it nextcloud-aio-nextcloud php occ config:system:set default_phone_region --value=“yourvalue”
Have a look at this site
and this one
https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/occ_command.html