mitchellh/vagrant — pull request #4905 by gildegoma (2014-12-07 10:12:44+00:00)
Title: providers/docker: fix support of agent forwarding

Without this change `vagrant ssh machine0` with the `:docker` provider won't forward ssh identity when `config.ssh.forward_agent = true`.

With the fix:

```
vagrant@machine0:~$ echo $SSH_AUTH_SOCK
/tmp/ssh-28S1mzZ9Ga/agent.355
```