This commit:

* binds the SSH agent socket (as found in $SSH_AUTH_SOCK) from the host
  to a predictable location inside the container;
* sets the SSH_AUTH_SOCK variable in the container to point to said
  predictable location.

This allows processes running inside the container to communicate with
the host's SSH agent and therefore to use the user's SSH keys without
having to expose the keys directly to the container.

closes #852