Doc: Added troubleshooting section for Nebulous docker errors

[tofarr]

Added troubleshooting for docker hash errors.

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Sometimes docker gives errors which seem to originate from outside openhands and pruning seems to clear things up. This documents that process.

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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:d877615-nikolaik   --name openhands-app-d877615   docker.all-hands.dev/all-hands-ai/openhands:d877615