Refactor Dockerfile for multi-stage build and update start script in package.json

- Introduced a multi-stage build in the Dockerfile to separate build and runtime environments.
- Added environment variables for production in the runtime stage.
- Updated the start script in package.json to point to the new server entry point.

package.json

@@ -7,7 +7,8 @@
     "dev": "nuxt dev",
     "generate": "nuxt generate",
     "preview": "nuxt preview",
-    "postinstall": "nuxt prepare"
+    "postinstall": "nuxt prepare",
+    "start": "node .output/server/index.mjs"
   },
   "dependencies": {
     "mongodb": "^6.18.0",