title: SSH agent forwarding during docker build
body: <p>While building up a docker image through a <code>dockerfile</code>, I have to clone a github repo. I added my public ssh keys to my git hub account and I am able to clone the repo from my docker host. While I see that I can use docker host's ssh key by mapping <code>$SSH_AUTH_SOCK</code> env variable at the time of docker run like</p>
  <pre><code>docker run --rm -it --name container_name \
    -v $(dirname $SSH_AUTH_SOCK):$(dirname $SSH_AUTH_SOCK) \
    -e SSH_AUTH_SOCK=$SSH_AUTH_SOCK my_image
  </code></pre>
  <p>How can I do the same during a docker build?</p>