Issue6: rename 'ports' into 'expose' and use 'ports' to publish

Hello David,

The keyword 'ports' is currently used in the yaml configuration file to expose some ports (to be able to link other containers)
If I am not mistaken, it corresponds to the "expose" keyword in docker and fig.

I propose the follwoing changes:

* keyword 'ports' is renamed into 'expose'
* a new 'ports' is added to publish the ports to the host
* the documentation is updated accordingly
* an example is added to test with a simple elasticsearch cluster