title: unable to find user root: no matching entries in passwd file
body: <p>I have a running Docker container with the following:</p>
  
  <pre><code>docker exec -it cc58908a8dc2 /bin/bash
  </code></pre>
  
  <p>I ran this command before on other containers and it worked just fine.
  Now, however, I'm getting this issue:</p>
  
  <pre><code>unable to find user root: no matching entries in passwd file
  </code></pre>
  
  <p>My <code>Dockerfile</code> has the following contents:</p>
  
  <pre><code># We will be running our Spark jobs as `root` user.
  USER root
  
  # Working directory is set to the home folder of `root` user.
  WORKDIR /root
  </code></pre>
  
  <p>Any idea of what could cause this?</p>