perf: ship a standalone slim Docker image (~590MB to ~283MB) (#841)

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.

next.config.mjs

@@ -16,6 +16,10 @@ const withMDX = nextMDX({
 
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+    // Emit a self-contained server (.next/standalone) with a traced, minimal
+    // node_modules, so the Docker image ships just the server + static assets
+    // instead of the full source and dependency tree.
+    output: 'standalone',
     assetPrefix: undefined,
     reactStrictMode: true,
     pageExtensions: ['js', 'jsx', 'ts', 'tsx', 'mdx'],