Add GitHub Copilot workspace instructions

[Copilot]

This PR adds comprehensive GitHub Copilot workspace instructions to improve AI-assisted development experience for the docker_ci_images repository.

What's Added

Creates a new .github/copilot-instructions.md file that provides GitHub Copilot with detailed context about:

• Project Overview: Clear explanation that this repository maintains Docker CI images for Cosmian's CI/CD pipeline
• Repository Structure: Visual breakdown of all Dockerfiles and their purposes
• Docker Images Documentation: Detailed specifications for each image:
  • cosmian/ubuntu22 - Ubuntu 22.04 with documentation tools (MkDocs, pandoc, LaTeX)
  • cosmian/rockylinux8 - Rocky Linux 8.9 with development tools
  • cosmian/rockylinux9 - Rocky Linux 9.3 with development tools
• Development Guidelines: Best practices for adding new images, Dockerfile optimization, and CI integration
• Local Development: Commands for building and testing images locally
• Troubleshooting: Common issues and solutions for build failures, size optimization, and CI problems

Why This Helps

The instructions enable GitHub Copilot to provide more accurate and contextually relevant suggestions when working with:

• Dockerfile modifications and optimizations
• CI/CD workflow updates
• Package management across different Linux distributions
• Docker image building and testing workflows

Impact

This is a purely additive change that doesn't modify any existing functionality. It enhances the development experience by providing AI assistants with comprehensive project context, leading to better code suggestions and reduced development friction.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.