Multiple external ports bound to a single internal port

Hi !

As I tested fig for my containers I realized that when using for ports declaration the following configuration :
```
  ports:
    - "80:80"
    - "5000:80"
```

Only the last binding was used.
It it due to the way the port_bindings dict is filled.
I changed that to allow this kind of configuration to work.

This is a resubmit of my previous PR ... hopefully cleaner for travis ;)