Provide the ability to bind to a Unix socket

[mikesir87]

I'm wanting to share the Docker socket with services that look for the Docker socket at /var/run/docker.sock. Since these apps need read-only access the proxy makes a lot of sense here. But, I'd prefer to leverage socket-based communication instead of publishing the port or relying on the container networking.

This PR provides the ability to bind the socket to a Unix socket, which can be put into a volume and shared with the other containers.