Add comprehensive GitHub Copilot instructions for defang-github-action

.github/copilot-instructions.md

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+# Defang GitHub Action
+
+Defang GitHub Action is a composite GitHub Action that enables deployment of applications using the [Defang CLI](https://defang.io/). The action supports deployment to both the Defang Playground and your own AWS account using Docker Compose-based projects.
+
+**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
+
+### Prerequisites and Setup
+- **No build step required** - This is a composite GitHub Action using shell scripts only
+- Node.js v20+ is required for testing the test application
+- Docker is required for container testing and builds
+- The action installs the Defang CLI automatically via curl script
+
+### Testing the Action
+- Run the test Node.js application:
+  ```bash
+  cd test
+  export PORT=3000
+  export DEFANG_GH_ACTION_TEST_MESSAGE="Hello from test"
+  node index.js
+  # Test with: curl http://localhost:3000
+  ```
+
+- Build and test the Docker container:
+  ```bash
+  cd test
+  docker build -t test-app .  # Takes ~20 seconds for initial build, use cached layers after
+  docker run --rm -d -p 3001:3000 -e PORT=3000 -e DEFANG_GH_ACTION_TEST_MESSAGE="Hello from Docker" test-app
+  # Test with: curl http://localhost:3001
+  docker stop $(docker ps -q --filter ancestor=test-app)
+  ```
+
+- Test with Docker Compose:
+  ```bash
+  cd test
+  docker compose up -d  # Uses cached image if already built
+  # Test with: curl http://localhost:3000
+  docker compose down
+  ```
+
+### Defang CLI Installation Testing
+**WARNING**: Defang CLI installation may fail due to GitHub API rate limits in testing environments.
+- The installation script downloads from GitHub releases: `curl -Lf https://raw.githubusercontent.com/DefangLabs/defang/main/src/bin/install`
+- Installation typically takes 10-30 seconds when successful
+- **NEVER CANCEL** the installation process - wait for completion or explicit error
+- In CI environments, the action sets `GH_TOKEN` to avoid rate limits
+
+### Running GitHub Action Tests
+- The CI workflow runs dry-run deployments to validate the action
+- Tests use `continue-on-error: true` because dry runs are expected to "fail"
+- Test scenarios:
+  1. Deploy with config environment variables
+  2. Deploy with empty parameters and staging mode
+  3. Teardown configuration
+
+Run tests locally using act or by checking out and testing individual components:
+```bash
+# Test the action steps manually (requires GITHUB_TOKEN)
+export GH_TOKEN="your-token"
+export DEFANG_PROVIDER="defang"
+export DEFANG_DEBUG="1"
+
+# This would normally be done by the action:
+. <(curl -Lf https://raw.githubusercontent.com/DefangLabs/defang/main/src/bin/install || echo return $?)
+defang login  # Requires interactive authentication
+defang whoami
+```
+
+## Validation
+
+### Manual Testing Requirements
+**CRITICAL**: Always manually validate changes by running complete scenarios:
+
+1. **Test the Node.js Application**:
+   ```bash
+   cd test
+   export PORT=3000 DEFANG_GH_ACTION_TEST_MESSAGE="Test message"
+   node index.js &
+   sleep 2
+   curl http://localhost:3000  # Should return "Test message"
+   kill %1
+   ```
+
+2. **Test Docker Build and Run**:
+   ```bash
+   cd test
+   docker build -t test-app .
+   docker run --rm -p 3001:3000 -e PORT=3000 -e DEFANG_GH_ACTION_TEST_MESSAGE="Docker test" test-app &
+   sleep 3
+   curl http://localhost:3001  # Should return "Docker test"
+   docker stop $(docker ps -q --filter ancestor=test-app)
+   ```
+
+3. **Test Action Configuration**:
+   - Verify `action.yaml` syntax is valid
+   - Ensure all input parameters are properly defined
+   - Test shell script syntax in the composite action steps
+
+### Timing Expectations
+- **Node.js app startup**: < 1 second
+- **Docker build (initial)**: 15-25 seconds - **NEVER CANCEL**
+- **Docker build (cached)**: < 5 seconds
+- **Defang CLI installation**: 10-30 seconds - **NEVER CANCEL**
+- **Container startup**: 2-5 seconds
+
+### Before Committing Changes
+**Always run these validation steps before committing**:
+
+1. Test the Node.js application works correctly
+2. Verify Docker builds complete successfully  
+3. Validate action.yaml syntax
+4. Test any shell script changes in the action steps
+5. Ensure the test compose files are valid
+
+**No traditional linting required** - this is a shell-script based GitHub Action.
+
+## Common Tasks
+
+### Repository Structure
+```
+.
+├── .github/
+│   └── workflows/
+│       └── test.yaml           # CI workflow that tests the action
+├── test/                       # Test application directory
+│   ├── Dockerfile             # Node.js container definition
+│   ├── compose.yaml           # Main compose file
+│   ├── compose.prod.yaml      # Production compose overlay
+│   └── index.js               # Simple HTTP server for testing
+├── action.yaml                # Main GitHub Action definition
+├── README.md                  # Usage documentation
+└── LICENSE                    # MIT license
+```
+
+### Key Action Inputs
+- `cli-version`: Defang CLI version (default: latest)
+- `config-env-vars`: Environment variables to deploy as config
+- `cwd`: Working directory (default: ".")
+- `compose-files`: Compose files to use (space-separated)
+- `mode`: Deployment mode (development/staging/production)
+- `provider`: Cloud provider (aws/defang/digitalocean)
+- `command`: Command to run (default: "compose up")
+
+### Action Workflow Steps
+1. **Install defang**: Downloads and installs Defang CLI
+2. **Set environment variables**: Configures provider and debug settings
+3. **Login to Defang**: Authenticates with Defang service
+4. **Set configuration**: Deploys environment variables as secure config
+5. **Run command**: Executes the specified Defang command
+
+### Testing Patterns
+The action is tested using dry-run deployments:
+- `defang compose up --dry-run` validates configuration without deployment
+- Multiple scenarios test different parameter combinations
+- Configuration cleanup ensures no state leakage between test runs
+
+### Common Issues and Solutions
+- **Rate limit errors**: Use `GH_TOKEN` environment variable for CLI installation
+- **Port conflicts**: Change port mappings when testing locally
+- **Authentication failures**: Defang CLI requires interactive login for actual deployments
+- **Compose file errors**: Validate YAML syntax and Docker Compose specification compliance
+
+### Development Workflow
+1. Make changes to action.yaml or test files
+2. Test locally using the validation steps above
+3. Verify CI workflow passes with your changes
+4. The action itself requires no build step - changes are immediately testable
+
+## Troubleshooting
+
+### If Tests Fail
+- Check action.yaml syntax using YAML validator
+- Verify shell script syntax in action steps
+- Test Docker builds complete successfully
+- Ensure test application responds correctly
+- Review CI logs for specific error details
+
+### If Defang CLI Installation Fails
+- Check GitHub API rate limits
+- Verify internet connectivity
+- Set GH_TOKEN environment variable
+- Wait for transient network issues to resolve
+
+This repository contains a working GitHub Action that requires no build process, only validation testing.