Functional port expose. Not a full port mapping implementation.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
I'm surprised this is required:

* There's this first loop on **exposed** ports which does `e.Expose(..., false)`
* There's a second loop right below on **published** ports which does `e.Expose(..., true)`

As long as the endpoint handles being called twice on a same port specification sequentially with `publish=false` then `publish=true`, this change should not be necessary.