Railway Build Cache Reuse vs Manual Prebuilt Image Deployments: Choosing Between Speed and Stale‑Cache Safety

When deploying a Railway service, teams can rely on the automatic build cache generated by Nixpacks to speed up successive deploys, or they can upload a prebuilt Docker image for each release to guarantee a clean artifact. The build cache improves iteration time but may retain previous build outputs when only environment variables or non‑build files change, potentially serving stale results. Conversely, manual image uploads eliminate that risk but lose the incremental speed benefit and require an external CI step.

Given the trade‑off between deploy speed and cache‑safety, what conditions cause Railway to invalidate its Nixpacks cache, and how should teams decide which approach to adopt for workloads that are sensitive to stale build artifacts?