title: Enable credentials in puckel/docker-airflow
body: <p>I am using <a href="https://hub.docker.com/r/puckel/docker-airflow/dockerfile" rel="nofollow noreferrer">puckel/docker-airflow</a> to deploy airflow. 
  Currently, the webserver is not asking for any credentials to login.
  How can I add a user to it? Maybe i have to add some environment variable in docker-compose.yml, but i am unable to find it. The docker-compose file is <a href="https://github.com/puckel/docker-airflow/blob/master/docker-compose-CeleryExecutor.yml" rel="nofollow noreferrer">here</a></p>
  
  <p>Thanks in advance.</p>