Add abstraction for Engine, Registry and Template APIs

[amihaiemil]

At the moment the Docker interface represents only the Engine API, yet Docker also offers Registry and Template APIs. See here.

We should make the Docker interface more abstract. It should offer 3 methods:

public interface Docker {
    Engine engine();
    Registry registry(); 
    Template template();
}

All the methods that are currently on the Docker interface should be moved on the Engine interface.

[amihaiemil]

N/A

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