Check NetworkSettings.Ports instead of HostConfig.PortBindings in tests

The latter appears to be unreliable.
Support 'expose' config option, like docker's --expose

Exposes ports to linked services without publishing them to the world.
Display unpublished ports in 'fig ps'
Remove unneeded 'ports' entries from WP and Django fig.ymls in docs
Merge pull request #143 from orchardup/expose-option

Support 'expose' config option, like docker's --expose