Add Woodpecker CI plugin for Dev Containers

[head1328]

This PR adds a new Woodpecker CI plugin that enables building and running Dev Containers in Woodpecker CI pipelines.

Closes #430

Changes

New Features

• Complete Woodpecker CI plugin implementation
• Docker image with all required dependencies (docker, buildx, skopeo, devcontainer CLI)
• Comprehensive documentation and examples
• Full test coverage with Jest (18 passing tests)
• Local development environment with Forgejo + Woodpecker CI

Plugin Capabilities

• Build Dev Container images with feature support
• Run commands inside containers (run_cmd)
• Multi-platform builds (linux/amd64, linux/arm64)
• Image caching and pushing to registries
• Environment variable passthrough
• Configurable paths and options

Code Quality

• TypeScript with strict mode
• ESLint configuration
• Prettier formatting
• Comprehensive unit tests
• CI pipeline with format-check, lint, build, and integration tests

Documentation

• Complete README with setup instructions
• Plugin marketplace documentation (docs.md)
• Example configurations for common use cases
• Troubleshooting guide

Testing

• All 18 unit tests passing
• Integration tests in CI pipeline
• Tested with multiple Dev Container configurations:
  • Simple Node.js (image-based)
  • Custom Dockerfile
  • Dev Container features
• Local testing with docker-compose (Forgejo + Woodpecker)

Breaking Changes

None - this is a new addition.

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