fix docker leak

[rbren]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below
  no changelog

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Give a summary of what the PR does, explaining any non-trivial design decisions

Docker images are getting left hanging because the atexit isn't registering properly. Needed to fix some imports to make it work

Here's the relevant atexit:

OpenHands/openhands/runtime/impl/eventstream/eventstream_runtime.py

Line 54 in 79ed4e3

atexit.register(remove_all_runtime_containers)

When we import inside of get_runtime_cls, this isn't registered in the main thread, so it gets left hanging

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Link of any specific issues this addresses

Fixes #4967

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:62c2a46-nikolaik   --name openhands-app-62c2a46   docker.all-hands.dev/all-hands-ai/openhands:62c2a46