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 sandboxes via Docker, which was missing from the docs. The AI agent added this documentation with the exact Docker command syntax that was provided in the issue.

2. The core maintainer (@li-boxuan) clarified that the functionality is already released and working, with the reported issues being related to macOS settings rather than a broken feature.

3. The added documentation includes:

   • The complete Docker command with all necessary parameters
   • Important setup requirements
   • Special notes about macOS configuration requirements, which directly addresses the user-reported issues

4. The changes directly fulfill the original request to "document it accordingly" with the Docker command that was provided in the issue description.

The implementation matches exactly what was requested in the issue - adding documentation for an existing feature that was previously only documented for development workflow. The fact that some users experienced configuration-related issues doesn't invalidate the documentation itself, as those issues were clarified to be platform-specific setup problems rather than feature bugs.

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:1e263ea-nikolaik   --name openhands-app-1e263ea   docker.all-hands.dev/all-hands-ai/openhands:1e263ea