Add Volumes to RuntimeContainer

This adds a `Volumes` field to the `RuntimeContainer` struct, consisting of a `map[string]Volume`.

This could probably benefit from some tests, but there weren't any existing `getContainer` tests to add to. We'll also want to document the changes to `RuntimeContainer` and the new `Volume` struct on [the wiki](https://github.com/jwilder/docker-gen/wiki/Docker-Gen-Emit-Structure).

Fixes #20.