docs: improve custom sandbox guide with more configuration options

[neubig]

This PR improves the custom sandbox guide by:

1. Clarifying that any base image can be used, not just Debian-based ones
2. Adding examples using both the default nikolaik/python-nodejs image and a Ruby-specific image
3. Adding a new section "Additional Configuration Options" that documents:
   • runtime_extra_deps for installing additional dependencies
   • runtime_startup_env_vars for setting environment variables
   • platform for multi-architecture builds

---

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:b8beaf6-nikolaik   --name openhands-app-b8beaf6   docker.all-hands.dev/all-hands-ai/openhands:b8beaf6