Add reusable action for building Docker images with Nix flakes:
- Full reproducibility with Nix derivations
- Attic cache integration for build artifacts
- Optimized layering with dockerTools.buildLayeredImage
- Automatic Nix binary cache usage
Use this instead of docker-build when you want:
- Bit-for-bit identical builds
- Better caching via Attic/Nix
- Smaller, optimized images
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
