Remove superfluous phantom data from container

[pbillaut]

The client_lifetime doesn't need to be tracked with PhantomData for docker_client to outlive the container itself and thus can be removed.

[thomaseizinger]

Hmm, including this was actually on purpose. However, I've since pondered over a different API and wrote up https://github.com/testcontainers/testcontainers-rs/issues/386. That would make this obsolete and also remove this lifetime.