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/.dockerignore

@@ -0,0 +1,6 @@
+node_modules
+.nuxt
+.output
+.git
+dist
+*.log