Fix issue #6134: Document how to use custom sandbox container with Docker

[openhands-agent]

This pull request fixes #6134.

The issue has been successfully resolved because:

1. The original issue specifically requested documentation for using custom sandbox containers via Docker, which was missing from the guide
2. The AI agent added a new "Using Docker" section that includes:
   • The complete Docker command with all required parameters exactly as specified in the issue
   • Explanations for how to specify custom images
   • Additional context about image compatibility
   • Important notes about macOS configuration requirements based on the thread context

The changes directly address the documentation gap identified in the issue by providing the missing Docker-specific instructions that were previously only available for the Development Workflow. The inclusion of the exact command syntax and supporting details ensures users have all necessary information to implement custom sandbox containers via Docker.

The solution is complete since this was purely a documentation issue and the requested content has been added in full, including the specific Docker command that was provided in the issue description.

Automatic fix generated by OpenHands 🙌

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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:25817ee-nikolaik   --name openhands-app-25817ee   docker.all-hands.dev/all-hands-ai/openhands:25817ee