Add agy cli to docker sbx

### What would you like to be added?

The docker sandboxes (sbx) documentation still [lists gemini cli](https://docs.docker.com/ai/sandboxes/agents/gemini/) with no mention of support for agy.

Can your team reach out to the Docker team so the correct cli is used?

I am using a 3rd party kit that enables agy but I think its something that should be in there out of the box and your two companies should get in touch and make this happen.

### Why is this needed?

Docker sandboxes (sbx) supports all other cli tools and is the go to solution for running ai agents safely. Agy should be represented instead of the old gemini cli.

### Additional context

_No response_

Start with the linked Docker Sandboxes Gemini CLI documentation and determine whether the requested change belongs in gemini-cli or in Docker's documentation. Confirm what agy support and ownership mean here; the issue is done only when the responsible team has clarified or updated the documented CLI support.

Written by the indexing model from the issue text.