Add GitHub Copilot workspace instructions

.github/copilot-instructions.md

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+# GitHub Copilot Workspace Instructions
+
+## Project Overview
+
+This repository contains Docker CI images used in the Cosmian CI/CD pipeline. The repository builds and maintains standardized Docker images for different operating systems used in continuous integration workflows.
+
+## Repository Structure
+
+```
+.
+├── .github/
+│   └── workflows/
+│       └── ci.yml              # GitHub Actions workflow for building and pushing images
+├── Dockerfile.ubuntu22         # Ubuntu 22.04 based image with documentation tools
+├── Dockerfile.rockylinux8      # Rocky Linux 8.9 based image with development tools
+├── Dockerfile.rockylinux9      # Rocky Linux 9.3 based image with development tools
+└── README.md                   # Basic project description
+```
+
+## Docker Images
+
+### Ubuntu 22.04 (`Dockerfile.ubuntu22`)
+- **Purpose**: Documentation and publishing pipeline
+- **Base**: ubuntu:22.04
+- **Key packages**: MkDocs, pandoc, LaTeX, Python documentation tools
+- **Registry**: `cosmian/ubuntu22`
+- **Use case**: Building documentation, generating PDFs, markdown processing
+
+### Rocky Linux 8 (`Dockerfile.rockylinux8`)
+- **Purpose**: Development environment for RHEL 8 compatible systems
+- **Base**: rockylinux:8.9
+- **Key packages**: GCC, development tools, GTK libraries
+- **Registry**: `cosmian/rockylinux8`
+- **Use case**: Building and testing applications on RHEL 8 compatible systems
+
+### Rocky Linux 9 (`Dockerfile.rockylinux9`)
+- **Purpose**: Development environment for RHEL 9 compatible systems
+- **Base**: rockylinux:9.3
+- **Key packages**: GCC, development tools, GTK libraries
+- **Registry**: `cosmian/rockylinux9`
+- **Use case**: Building and testing applications on RHEL 9 compatible systems
+
+## Development Guidelines
+
+### Adding New Docker Images
+
+1. **Naming Convention**: Use `Dockerfile.{os}{version}` format
+2. **Base Image Selection**: Use specific version tags with SHA256 hashes for reproducibility
+3. **Package Installation**: 
+   - Group related packages in single RUN commands to reduce layers
+   - Use `--no-install-recommends` for apt-get to minimize image size
+   - Clean package caches after installation
+4. **CI Integration**: Add new images to the matrix in `.github/workflows/ci.yml`
+
+### Dockerfile Best Practices
+
+- **Layer Optimization**: Combine related commands in single RUN statements
+- **Security**: Use specific base image versions with SHA256 hashes where possible
+- **Reproducibility**: Pin package versions when critical for builds
+- **Documentation**: Comment complex installations or workarounds
+
+### CI/CD Workflow
+
+The GitHub Actions workflow (`.github/workflows/ci.yml`):
+- Triggers on every push to any branch
+- Builds all Docker images in parallel using matrix strategy
+- Pushes to Docker Hub registry under `cosmian/` namespace
+- Requires `DOCKER_HUB_PWD` secret for authentication
+
+### Modifying Existing Images
+
+1. **Test Changes**: Build locally before committing
+2. **Version Considerations**: Understand impact on dependent CI pipelines
+3. **Documentation**: Update this file if adding significant new capabilities
+4. **Validation**: Ensure images build successfully in CI
+
+### Local Development
+
+```bash
+# Build a specific image locally
+docker build -f Dockerfile.ubuntu22 -t test-ubuntu22 .
+
+# Test image functionality
+docker run --rm -it test-ubuntu22 /bin/bash
+
+# Check installed packages
+docker run --rm test-ubuntu22 dpkg -l  # for Ubuntu
+docker run --rm test-rockylinux8 rpm -qa  # for Rocky Linux
+```
+
+## Common Tasks
+
+### Adding Python Packages (Ubuntu image)
+- Add to the pip3 install command in the existing RUN layer
+- Consider using requirements.txt for complex dependency management
+
+### Adding System Packages
+- **Ubuntu**: Add to existing `apt-get install` command
+- **Rocky Linux**: Add to existing `dnf install` command
+- Remember to update package cache cleanup commands
+
+### Updating Base Images
+- Check for security updates regularly
+- Update SHA256 hashes when changing base image versions
+- Test thoroughly as base image changes can have wide impact
+
+## Troubleshooting
+
+### Build Failures
+- Check package availability for target OS version
+- Verify package names (they may differ between distributions)
+- Review dependency conflicts
+
+### Size Issues
+- Use multi-stage builds if needed
+- Combine RUN commands to reduce layers
+- Remove unnecessary files in the same layer they're created
+
+### CI Issues
+- Verify Docker Hub credentials and permissions
+- Check matrix configuration syntax
+- Ensure all referenced Dockerfiles exist
+
+## Integration Notes
+
+These images are designed to be consumed by other Cosmian projects' CI pipelines. Changes should be coordinated with teams using these images to avoid breaking dependent builds.