IOT Sample Project: Add user to mdash (on premise)

I’m currently evaluating the mongoose-os-smart-light example. I’m trying to add a second demo user to the mdash instance running within docker on my computer. However, using the example code from the readme section Backend, I can not add any user:

jakob@host:~/mos/mongoose-os-smart-light-master/backend$ cat data/db.json 
{
  "users": {},
  "devices": {},
  "apikeys": {},
  "billings": {},
  "apps": {},
  "autoconfs": {}
}jakob@host:~/mos/mongoose-os-smart-light-master/backend$ docker-compose run dash /dash --config-file /data/dash_config.json --register-user user password
2020/03/06 15:44:07 dash.go:1945: DB saved to /data/db.json, backup /data/backups/db.json.2020-03-06.15:44:07
jakob@host:~/mos/mongoose-os-smart-light-master/backend$ cat data/db.json 
{
  "users": {},
  "devices": {},
  "apikeys": {},
  "billings": {},
  "apps": {},
  "autoconfs": {}
}