Do not always stop container on __del__

[andredasilvapinto]

A user of the library might, for examle, not keep the reference to DockerContainer but use get_wrapped_container to access the underlying Container object and keep that instead. Currently this will produce an unexpected container stop triggered by GC.

If you want to keep this behaviour it should at least be optional (configured by a constructor stop_on_deletion flag for example).

Relevant code: https://github.com/testcontainers/testcontainers-python/blob/master/testcontainers/core/container.py#L69-L77

[tillahoffmann]

Duplicate of #109. Let's discuss further there if needed.