Replaced procfile with heroku.yml, added heroku-only Dockerfile and updated app.json

Dockerfile.heroku

@@ -0,0 +1 @@
+FROM tzahi12345/youtubedl-material:nightly

Procfile

@@ -1 +0,0 @@
-web: npm start --prefix backend

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"]
   }

heroku.yml

@@ -0,0 +1,3 @@
+build:
+  docker:
+    web: Dockerfile.heroku