Add Docker::Container::Config class

The Docker API, as robust as it is, is not supremely documented and it can be difficult for users to know the small details necessary to configure it correctly. However, most users know how to use the CLI. Instances of the `Docker::Container::Config` class can be used to build up the hash you pass in to `Docker::Container.create` using methods and parameters that match what you typically use in the CLI. 

The motivation for this class was that in my projects, the configuration is specified by end users that may or may not know how to use the API. This way they can provide the configuration in a way that is familiar to them. 
`-e ENV=variable` won't fit in here.