How do I mount a directory from the host machine to a container in Docker?
Just as a final update, this feature is now released in Docker (though the API has changed since the pull request linked by @imiric).
Simply use a command like
docker run -v /tmp:/root myImage
in order to mount /tmp
from the host machine as /root
within the image.
Source: https://docs.docker.com/engine/userguide/dockervolumes/