To add passwords, create a passwords.txt file - do not edit the passwords.mqtt directly. Info in passwords.README file.
The way I use it - create the /config directory and everything should be setup.
Stop the docker, create a passwords.txt and then start the docker.
It will encrypt the passwords, add it to the top of passwords.mqtt and then delete passwords.txt.
If you have to use the cmdline, try the following:
docker run -t -i --net="bridge" -p 1883:1883/tcp -p 9001:9001 -v /mnt/cache/app_config/mqtt/:/config spants/mqtt
You can stop the container and edit mosquitto.conf to remove password requirement