While building up a docker image through a `dockerfile`, 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 `$SSH_AUTH_SOCK` env variable at the time of docker run like

```
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

```

How can I do the same during a docker build?