Add 'nodejs' buildback (Ref: rails/webpacker#739)

Procfile

@@ -1,2 +1,3 @@
-rails: rails server -p 3333
-webpack: ./bin/webpack-dev-server
+release: bin/rails db:migrate
+worker: NODE_ENV=production bin/rails webpacker:compile
+web: bin/rails server -p $PORT -e $RAILS_ENV

Procfile.dev

@@ -0,0 +1,2 @@
+rails: rails server -p 3333
+webpack: ./bin/webpack-dev-server

app.json

@@ -2,8 +2,7 @@
   "name": "elmsanity",
   "description": "",
   "scripts": {
-    "postdeploy": "cp config/database.yml.postgresql config/database.yml; bundle exec rails db:create; bundle exec rails db:migrate; NODE_ENV=production bundle exec rails webpacker:compile"
-  },
+},
   "env": {
     "SECRET_KEY_BASE": {
       "generator": "secret"
@@ -14,11 +13,14 @@
     "HEROKU": "true"
   },
   "formation": {
+    "web": { "quantity": 1 },
+    "worker": { "quantity": 1 }
   },
   "addons": [
 
   ],
   "buildpacks": [
-
+    { "url": "heroku/nodejs" },
+    { "url": "heroku/ruby" }
   ]
 }

package.json

@@ -5,10 +5,10 @@
     "@rails/webpacker": "^3.0.2",
     "coffeescript": "1.12.7",
     "elm": "^0.18.0",
-    "elm-webpack-loader": "^4.4.0"
+    "elm-webpack-loader": "^4.4.0",
+    "webpack-dev-server": "^2.9.4"
   },
   "devDependencies": {
-    "elm-hot-loader": "^0.5.4",
-    "webpack-dev-server": "^2.9.4"
+    "elm-hot-loader": "^0.5.4"
   }
 }