title: Docker image - add user to group
body: <p>I have an custom docker image, already builded. There is no Dockerfile available. Inside container, instead of root, an custom user, let's say <em>test</em>, is available. This user is attached to group <em>test</em>. This is default user for container.</p>
  <p>I try to build my own image using described above base image, and add user <em>test</em> also to group <em>workers</em>. How to do this?</p>
  <p>Using known commands (useradd, addgroup, usermod) known by me, is not helpful.
  After <em>docker exec</em> inside container, command <em>id</em> returns me only one group (<em>test</em>).</p>
  <p>Also access to files mounted to container with permissions <em>anotheruser:workers</em> / 770 is not possible.</p>
  <p>Is there a chance to do this?</p>