Docker root passwd not changed

I have pulled ubuntu 16.04. and create a container using below command:

 docker run -it -d --name ubuntu  --privileged ubuntu:16.04

change root password of ubuntu_container, after that ssh to this ubuntu_container, but docker root password not setting. its throw “Permission denied, please try again”

I have go to ubuntu container:

    root@efk:/# passwd 
        Enter new UNIX password: 
        Retype new UNIX password: 
        passwd: password updated successfully

Suggest Me How to Change Docker Container Root passwd and default ubuntu Container root Password.
Why this issue occur.