Volume mounting bug fix in docker_backend.go.

Signed-off-by: Joao Mares <joaomares@gmail.com>
I was trying to mount a host directory inside a runner by adding to job.Config.Mounts the host directory and the target directory in the container. I spoke with lmars because the directory was showing up in the container binds but there was no actual directory mounted inside the container and this fix was what he suggested to me.