build(website): Dockerfile + production start script for Coolify

Make the marketing site deployable standalone from apps/website (the repo
has no root package.json, so the build must run in this subdir — set Base
Directory to apps/website in Coolify).

- Add a multi-stage Dockerfile (pnpm build -> node .output/server/index.mjs,
  port 3000, NUXT_PUBLIC_SITE_URL default) + .dockerignore.
- Add a "start" script for the Nixpacks path (Nuxt SSR has no default start).
- Guard the bind-mount-only /shared-packages components dir with existsSync
  so standalone builds don't warn on the missing path (the site uses no
  shared component).

apps/website/package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "dev": "TMPDIR=/tmp nuxt dev --host 0.0.0.0 --port 3000",
     "build": "nuxt build",
+    "start": "node .output/server/index.mjs",
     "generate": "nuxt generate",
     "preview": "nuxt preview",
     "typecheck": "nuxt typecheck",