feat: Builder system with plugin support

[nfebe]

Summary

Introduce a “builder” concept so each app directory can define pre-deployment steps. Builders can be built-in or plugin-based. Some apps may use ready images, while others will be built from source (e.g., GitHub) using a builder.

Why

Certain apps require preparation before running—compiling assets, generating keys, running migrations, packaging binaries, etc. Builders standardize this workflow in FlatRun.

Requirements

• Builder Logic
  Encapsulate steps for specific app types. Examples:

  • npm run build / npm run dist → JS frontends
  • php artisan key:generate → Laravel
  • go build → Go projects
  • composer install → PHP apps

• Detection

  • Explicit config in app.yml (e.g., builder: "npm")
  • Auto-detect from files (package.json, composer.json, etc.)

• Plugin Support

  • Allow custom builders via modules or external hooks
  • Interface:
    • Detect(appDir string) bool
    • Build(appDir string) error
    • Optional: Prepare(appDir string), PostBuild(appDir string)

• Execution & Deployment Types

  • Run in sandboxed environment
  • Clear logs via Agent UI/API
  • Failures block deployment unless overridden
  • Support dry-run mode to preview commands
  • Deployment types:
    • Ready compose files → use existing Docker images
    • Build from source and generate compose files → use a builder to generate deployment artifacts