title: what is 'z' flag in docker container's volumes-from option?
body: <p>While going through the docker docs, I came across volumes-from (<a href="https://docs.docker.com/engine/reference/commandline/run/" rel="noreferrer">https://docs.docker.com/engine/reference/commandline/run/</a>)   option for docker run command. 
  I didn't understand the differences between <code>ro, rw, and z</code> option provided as-<br>
   <code>$ docker run --volumes-from ba8c0c54f0f2:ro -i -t ubuntu pwd</code><br>
  In the above command the <code>ro</code> option is replaced with <code>z</code>. I will be thankful if anyone explores on differences of using these options.</p>