Could not open a connection to your authentication agent.
I am following the approach of mounting the $SSH_AUTH_SOCK as a volume, but doing so with compose.
The previous accepted answer using whilp/ssh-agent did not work for me for some reason (it worked before but since last changes it doesn't and I don't know why) so I created my own agent container:
docker-ssh-agent
based on minimal alpine:3.4
base image. So anyone still having trouble with this on OSX, check the README
it's now really easy to get it up and running!