Update base image documentation with latest Dockerfile and security features

[codegen-sh]

This PR updates the base image documentation to reflect the current state of the sandbox Docker image from the cloud repo and re-adds it to the docs navigation.

Changes Made

Updated Documentation

• Updated base-image.mdx with the complete and current Dockerfile from the cloud repo
• Added new security features documentation including semgrep, trufflehog, and pre-push hooks
• Added missing tools like Homebrew, code-server, tree, and additional development utilities
• Updated environment variables and PATH configuration to match current implementation

Navigation Fix

• Re-added base-image to docs.json under the Sandboxes group as requested

Key New Features Documented

Security Tools

• semgrep for static analysis and secret detection
• trufflehog for credential scanning (via Homebrew)
• Pre-push git hooks for automated security checks

Additional Development Tools

• Homebrew package manager for additional system packages
• code-server for VS Code-like browser-based editing
• tree for directory visualization
• uvicorn for Python web server development

Enhanced Environment

• Updated PATH to include Homebrew binaries
• Added linuxbrew user for proper Homebrew operation
• Enhanced environment variable configuration

The documentation now accurately reflects the current sandbox environment and provides users with a complete understanding of available tools and capabilities.

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✅ All tests successful. No failed tests found.

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