add "image" key for heroku-docker

.gitignore

@@ -1,2 +1,4 @@
 node_modules
 .env
+Dockerfile
+docker-compose.yml

app.json

@@ -3,5 +3,6 @@
   "description": "A barebones Node.js app using Express 4",
   "repository": "https://github.com/heroku/node-js-getting-started",
   "logo": "http://node-js-sample.herokuapp.com/node.svg",
-  "keywords": ["node", "express", "static"]
+  "keywords": ["node", "express", "static"],
+  "image": "heroku/node"
 }