--userpasswd user:password
           Sets password for the given user in a container, creating the user if it does not exists.  Note that this option is not  saved  in
           configuration  file  at  all  (so  --save  flag  is  useless),  it  is  applied to the container (by modifying its /etc/passwd and
           /etc/shadow files).

           In case container root filesystem is not mounted, it is automatically mounted, then all the appropriate file changes are  applied,
           then it is unmounted.

           Note that container should be created before using this option.