Use Railpack's install env handling and dedicated CLI flags for build/start overrides, and forward install commands into docker build secrets so image builds stay aligned with prepare-time configuration. Update the railpack config test to cover the new command format.
- Implement railpack.json + generated config deep merging logic in
ApplicationDeploymentJob with JSON validation and assoc array checks
- Label Railpack as "Beta" in all build pack selectors and show a
visible beta badge when railpack is selected in new-app forms
- Add railpack-nodejs Fastify example to ApplicationSeeder
- Add ApplicationSeederTest and ApplicationDeploymentRailpackConfigTest
covering config merge behavior and seeder correctness
