Nixpacks Build Plan
The Nixpacks build plan is the JSON representation of how a source directory will be turned into a container image. A plan declares the list of providers used, top-level `variables`, `staticAssets`, `buildImage`, and an ordered set of phases (`setup`, `install`, `build`, plus any provider- or user-defined phases) along with a `start` phase. Each phase carries `cmds`, `nixPkgs`, `nixLibs`, `nixOverlays`, `nixpkgsArchive`, `aptPkgs`, `dependsOn`, `cacheDirectories`, `onlyIncludeFiles`, and `paths`. Plans can be saved and replayed via `nixpacks build --plan plan.json` to guarantee reproducible image builds.