title: Cannot create directory. Permission denied inside docker container
body: <p>Can not create folder during image building with non root user added to sudoers group. Here is my Dockerfile:</p>
  
  <pre><code>FROM ubuntu:16.04
  
  RUN apt-get update &amp;&amp; \
      apt-get -y install sudo
  
  RUN adduser --disabled-password --gecos '' newuser \
      &amp;&amp; adduser newuser sudo \
      &amp;&amp; echo '%sudo ALL=(ALL:ALL) ALL' &gt;&gt; /etc/sudoers
  
  USER newuser
  
  RUN mkdir -p /newfolder
  WORKDIR /newfolder
  </code></pre>
  
  <p>I get error: <code>mkdir: cannot create directory '/newfolder': Permission denied</code></p>