NEXT_PUBLIC_* and next.config rewrites are baked in at build time

Setting NEXT_PUBLIC_API_URL as a runtime env var in compose does nothing. Next inlines NEXT_PUBLIC_* at compile time and serialises rewrites() from next.config.ts into .next/routes-manifest.json; next start reads the manifest and never re-evaluates the config. An image built without the value as a --build-arg proxies to whatever the fallback was (often localhost:8000) forever, and the UI comes up empty with no error — it looks like an API outage, not a build misconfiguration. Architectural consequence worth stating in docs: this makes the frontend image per-deployment rather than one image configured at run time. Also watch for rewrites that strip a path prefix, which changes what the env var must contain.