I would like to use ssh-agent to forward my keys into the docker image and pull from a private github repo.
I am using a slightly modified version of https://github.com/
By default, boot2docker shares only files under /Users. SSH_AUTH_SOCK is probably under /tmp so the -v mounts the agent of the VM, not the one from your mac.
/Users
SSH_AUTH_SOCK
/tmp
-v
If you setup your VirtualBox to share /tmp, it should be working.