Infra: Ensure sandbox image is built with Release process

[mattKorwel]

This task is to migrate the hosting of our public sandbox Docker images from the private Google Artifact Registry to Docker Hub.

The Problem

Currently, our sandbox images are intended for public consumption but are hosted in a private Google Artifact Registry (us-west1-docker.pkg.dev/gemini-code-dev/...). This is not an ideal public distribution point because:

• Discoverability: Docker Hub is the default and most widely used container registry. Hosting our images there makes them easier for users to find and trust.
• Accessibility: While Artifact Registry can be configured for public access, it's not the standard workflow for most Docker users and can add unnecessary friction.
• Community Standard: Using Docker Hub aligns with open-source best practices and community expectations for public Docker images.

The Goal

To improve accessibility and align with community standards, we will publish our official sandbox images to a public repository on Docker Hub.

This involves:

1. Creating a Docker Hub Repository: Create an official public repository for the sandbox image under the @google organization on Docker Hub. This is a multi-step process that requires coordination. Please ping @mattkorwel for information on the internal process for requesting a new repository under this organization.
2. Updating the Release Pipeline: Modify our release workflow to authenticate to Docker Hub (using a token stored as a GitHub secret) and push the final image to the new repository.
3. Updating Documentation: Ensure any documentation or user-facing materials are updated to reference the new Docker Hub image path.

This change will make our sandbox environment more accessible and discoverable for our users and the wider open-source community.