Next.js rewrites and NEXT_PUBLIC_* are frozen at build time, not read at runtime

Containerizing a Next app and passing NEXT_PUBLIC_API_URL via docker run or compose environment: silently does nothing. NEXT_PUBLIC_* is inlined into the bundle at compile time, and next.config.ts rewrites() is serialized into .next/routes-manifest.json during next build — next start reads that manifest and never re-evaluates next.config.ts. The app then proxies to whatever fallback the config had at build time (often localhost), and the UI comes up empty with no error anywhere. Fix is ARG + ENV before RUN npm run build, so the values belong in compose build.args, not environment. Verify by grepping .next/routes-manifest.json inside the built image for the expected origin — the manifest is the ground truth, not the env. Consequence worth stating in any docs: the frontend image becomes per-deployment rather than one artifact configured at runtime.