title: "useradd: invalid user name" in Ansible user module
body: <p>
  My Ansible playbook:</p>
  
  <pre><code>- hosts: docker
    remote_user: root
  
    tasks:
    - include_vars: users.yml
    - name: Add users to local docker groups 
      user: name='{{users}}' groups='{{group_name}}' append=yes
  </code></pre>
  
  <p><code>users.yml</code> file:</p>
  
  <pre><code>users: 
      - jjohnson
  
  group_name: docker 
  </code></pre>
  
  <p><code>ansible-playbook docker-groups.yml</code>:</p>
  
  <pre><code>PLAY [docker] **********************************************************************************************************************************************************************************************
  
  TASK [Gathering Facts] *************************************************************************************************************************************************************************************
  ok: [server]
  
  TASK [include_vars] ****************************************************************************************************************************************************************************************
  ok: [server]
  
  TASK [Add users to local docker groups] ********************************************************************************************************************************************************************
  fatal: [server]: FAILED! =&gt; {"changed": false, "failed": true, "msg": "useradd: invalid user name '['jjohnson']'\n", "name": "['jjohnson']", "rc": 3}
  </code></pre>
  
  <p>Why is it falling?</p>