Expose `--network-alias` command line switch

When setting up an embedded mattermost via omnibus-gitlab we need to configure the gitlab container with a sense that it is both gitlab.example.com and mattermost.example.com so that the rails application can find the co-located mattermost API by name.

Here we add the code to transform a network_aliases array into multiple --network-alias arguments.

https://docs.docker.com/engine/reference/run/#network-settings

Towards #26 (closed)