title: `docker cp` doesn't copy file into container
body: <p>I have a dockerized project. I build, copy a file from the host system into the docker container, and then shell into the container to find that the file isn't there. How is <code>docker cp</code> supposed to work?</p>
  
  <pre><code>$ docker build -q -t foo .
  Sending build context to Docker daemon    64 kB
  Step 0 : FROM ubuntu:14.04
   ---&gt; 2d24f826cb16
  Step 1 : MAINTAINER Brandon Istenes &lt;redacted@email.com&gt;
   ---&gt; Using cache
   ---&gt; f53a163ef8ce
  Step 2 : RUN apt-get update
   ---&gt; Using cache
   ---&gt; 32b06b4131d4
  Successfully built 32b06b4131d4
  $ docker cp ~/.ssh/known_hosts foo:/root/.ssh/known_hosts
  $ docker run -it foo bash
  WARNING: Your kernel does not support memory swappiness capabilities, memory swappiness discarded.
  root@421fc2866b14:/# ls /root/.ssh
  root@421fc2866b14:/#
  </code></pre>