title: build context for docker image very large
body: <p>I have created a couple different directories on my host machine as I try to learn about Docker just to keep my dockerfiles organized.  My Dockerfile I just ran looks like this:</p>
  
  <pre><code>FROM crystal/centos
  MAINTAINER crystal
  
  ADD ./rpms/test.rpm ./rpms/ 
  RUN yum -y --nogpgcheck localinstall /rpms/test.rpm 
  </code></pre>
  
  <p>My actual rpm is only 1 GB.  But when I try to do <code>sudo docker build -t="crystal/test" .</code>, I get sending build context to Docker daemon 3.5 GB.  Is there something else that I'm unaware of as you continue to build Docker images? Is my memory accumulating as I build more images in my other directories on my host machine? </p>