Add comprehensive GitHub Copilot instructions for pycomposefile development

.github/copilot-instructions.md

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+# pycomposefile
+
+pycomposefile is a Python library for structured deserialization of Docker Compose files. It provides a structured way to parse, validate, and manipulate Docker Compose YAML configurations in Python applications.
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+## Working Effectively
+
+### Bootstrap and Setup
+- **Required Python version**: 3.9 or higher (tested with 3.10-3.14)
+- **Network Dependencies**: Most pip commands require PyPI access and may timeout in sandboxed environments
+- **Initial setup** (if pip access works): `python -m pip install -U pip` then `python -m pip install --group dev`
+- **Dependencies**: pyyaml>=6.0, build>=0.7, ruff==0.12.5, timeout-decorator==0.5.0
+- **If network issues**: Dependencies may already be installed; verify with `python -m ruff --version`
+
+### Development Workflow Commands
+- **Lint code**: `python -m ruff check` (takes <1 second) - **ALWAYS run before committing**
+- **Format code**: `python -m ruff format` (takes <1 second) - Auto-formats Python code 
+- **Check formatting**: `python -m ruff format --check` (takes <1 second) - **WARNING: Currently shows 31 files would be reformatted - this is normal for this codebase**
+- **Run tests**: `python -m unittest discover -v -s . -p "test*.py"` (takes <1 second, 85 tests, 1 typically skipped)
+- **Build package**: `python -m build` - **NETWORK DEPENDENT: Will fail with PyPI timeout in sandboxed environments. Use manual testing instead.**
+
+### Key Project Structure
+```
+/pycomposefile/           # Main library code
+  __init__.py            # Exports ComposeFile class
+  compose_file.py        # Core ComposeFile class
+  compose_element/       # Compose element parsing utilities
+  service/              # Service-specific parsing
+  secrets/              # Secret management
+/tests/                  # Test suite (unittest framework)
+  compose_file/         # Tests for compose file parsing
+  service/              # Tests for service parsing
+  environment_variable_evaluation/  # Tests for env var handling
+  compose_generator/    # Test data generators
+/sample/                 # Sample files for testing
+  test.env, common.env  # Environment files for testing
+  my_secret.txt         # Sample secret file
+  apps/web.env          # Application-specific env vars
+```
+
+## Validation
+
+### Manual Functionality Testing
+Always test the core library functionality after making changes:
+
+```python
+import sys
+sys.path.insert(0, '.')
+from pycomposefile import ComposeFile
+import yaml
+
+# Test basic parsing
+compose_yaml = '''
+version: '3'
+services:
+  web:
+    image: nginx:latest
+    ports:
+      - '80:80'
+  db:
+    image: postgres:13
+    environment:
+      POSTGRES_DB: myapp
+'''
+parsed = yaml.load(compose_yaml, Loader=yaml.Loader)
+compose_file = ComposeFile(parsed)
+print(f'Version: {compose_file.version}')
+print(f'Services: {list(compose_file.services.keys())}')
+```
+
+### Test Environment Variable Processing
+```python
+from tests.compose_generator import ComposeGenerator
+# Test environment files
+compose = ComposeGenerator.get_compose_with_environment_file_list()
+# Test service dependencies  
+compose = ComposeGenerator.get_compose_with_service_dependencies()
+# Test secrets
+compose = ComposeGenerator.get_compose_with_one_secret()
+```
+
+### CI Validation Steps
+Always run these commands before committing (matches .github/workflows/build.yaml):
+1. `python -m ruff check` - Must pass with no errors  
+2. `python -m unittest discover -v -s . -p "test*.py"` - All tests must pass (85 tests, 1 may be skipped)
+
+**Note on formatting**: `python -m ruff format --check` shows 31 files would be reformatted, which is the current state of the codebase. Focus on ensuring your changes don't introduce new linting errors.
+
+**Build command limitations**: The `python -m build` command requires network access to PyPI and will timeout in sandboxed environments. Use manual functionality testing instead.
+
+## Important Development Notes
+
+### Network Dependencies
+- **PyPI Access Required**: Package building and some pip commands require downloading from PyPI
+- **Timeout Issues**: In sandboxed environments, pip and build commands frequently fail with "Read timed out" errors
+- **Workarounds**: 
+  - Test core functionality directly with Python imports instead of relying on package building
+  - Dependencies are often pre-installed in development environments
+  - Focus on linting, testing, and manual functionality validation
+  - Use `python -m ruff --version` to verify ruff is available
+
+### Code Quality Requirements
+- **Linting**: All code must pass `ruff check` without warnings
+- **Formatting**: Code must be formatted with `ruff format` 
+- **Testing**: All new functionality must have corresponding unittest tests
+- **Coverage**: Tests are comprehensive with 85+ test cases covering all major functionality
+
+### Common File Locations
+- **Configuration**: `pyproject.toml` (build config, dependencies, ruff settings)
+- **Main API**: `pycomposefile/__init__.py` (exports ComposeFile)
+- **Core Logic**: `pycomposefile/compose_file.py` (main parsing logic)
+- **Test Data**: `tests/compose_generator/__init__.py` (ComposeGenerator with test YAML)
+- **Sample Files**: `sample/` directory (test.env, common.env, my_secret.txt)
+
+### Testing Framework Details
+- **Framework**: Python unittest (not pytest)
+- **Test Discovery**: Use `python -m unittest discover -v`
+- **Test Organization**: Tests are organized by functionality in separate directories
+- **Test Data**: ComposeGenerator class provides Docker Compose YAML snippets for testing
+- **Expected Results**: 85 tests run, typically 1 skipped, all others should pass
+
+### Environment & Dependencies
+- **Build Backend**: flit_core (defined in pyproject.toml)
+- **Main Dependency**: pyyaml for YAML parsing
+- **Dev Dependencies**: build, ruff, timeout-decorator
+- **Python Support**: 3.9, 3.10, 3.11, 3.12, 3.13, 3.14 (per CI matrix)
+
+### Development Container Support
+- **DevContainer**: `.devcontainer/devcontainer.json` with Python 3 setup
+- **Post-Create Script**: `.devcontainer/postCreate.sh` sets up virtual environment
+- **VSCode Config**: Configured for Python development with unittest integration
+
+Always validate your changes by running the full development workflow: install dependencies, lint, format-check, and run tests.