Enable Next `output: 'standalone'` and rewrite the Dockerfile to package only the traced standalone server + .next/static + public on node:20-slim (glibc, matching the runner), running `node server.js`. No in-image npm ci, no full source or dependency tree. Cuts the image ~52%, so docker push and the server pull get much faster.
Also fixes a latent entrypoint bug (${VAR:"none"} -> ${VAR:-"none"}) that busybox sh tolerated but Debian dash (node:20-slim /bin/sh) rejects, which would otherwise crash the container on boot.
Smoke-tested via isolated build + docker run: boots in 37ms, all routes 200, redirects still bundled (/slack-url -> 307), static + public assets served, DocSearch env-injection intact.
