Use Groovy @Delegate to Facades for testcontainers DockerClients

[kiview]

The Facade classes might still be useful in the future for allowing easier API changes or more idiomatic Groovy code without having to change code upstream. Still we want to expose the already existing functionalities of the testcontainers API.

It seems sensible to expose them via Groovy's @DeleGate field annotation.

[kiview]

This won't be needed after merging first-class-testcontainers branch anymore.