Added back stack field in app.json

app.json

@@ -2,6 +2,7 @@
     "name": "YoutubeDL-Material",
     "description": "An open-source and self-hosted YouTube downloader based on Google's Material Design specifications.",
     "repository": "https://github.com/Tzahi12345/YoutubeDL-Material",
+    "stack": "container",
     "logo": "https://i.imgur.com/GPzvPiU.png",
     "keywords": ["youtube-dl", "youtubedl-material", "nodejs"]
   }