title: Mounting file system in docker fails sometimes
body: <p>I'm trying to do something pretty trivial with Docker, and it seems to work sometimes, but not each time I try it. The command</p>
  
  <pre><code>docker run -v /Users/:/mnt/users/ ubuntu ls /mnt/users
  </code></pre>
  
  <p>works when I run it first time, but repeating it may give either </p>
  
  <pre><code>docker: Error response from daemon: mkdir /Users: file exists.
  </code></pre>
  
  <p>or sometimes </p>
  
  <pre><code>docker: Error response from daemon: Bad response from Docker engine.
  </code></pre>
  
  <p>response. Sorry if I'm missing something trivial, but I cannot imagine this is expected behaviour?</p>
  
  <p>Running OS X 10.11.5 and docker version 1.12.0-rc3</p>