Add NetworkMode to RuntimeContainer

[anthonyraymond]

If a container uses the network_mode: container:xxxx, both the .Networks and .Addresses attributes are empty.

Making NetworkMode accessible will be a first step in resolving issue in https://github.com/jwilder/nginx-proxy where people have to find workaround to alter the nginx .conf file to populate upstreams when they are routing their container network though an other container.

[anthonyraymond]

Any hope for this to be merged before a new release @jwilder ?

[viranch]

2 years later, I still want this!

[buchdag]

Superseded by #574