title: Why does simple Dockerfile give "permission denied"?
body: <p>I am learning to use Docker with ROS, and I am surprised by this error message:</p>
  
  <pre><code>FROM ros:kinetic-robot-xenial
  
  # create non-root user
  ENV USERNAME ros
  RUN adduser --ingroup sudo --disabled-password --gecos "" --shell /bin/bash --home /home/$USERNAME $USERNAME
  RUN bash -c 'echo $USERNAME:ros | chpasswd'
  ENV HOME /home/$USERNAME
  USER $USERNAME
  
  RUN apt-get update
  </code></pre>
  
  <p>Gives this error message</p>
  
  <pre><code>Step 7/7 : RUN apt-get update
   ---&gt; Running in 95c40d1faadc
  Reading package lists...
  E: List directory /var/lib/apt/lists/partial is missing. - Acquire (13: Permission denied)
  The command '/bin/sh -c apt-get update' returned a non-zero code: 100
  </code></pre>