fix app.json extraction in dockerfile apps not using /app. closes #2300

tests/apps/dockerfile/Dockerfile

@@ -4,8 +4,8 @@ EXPOSE 3001/udp
 EXPOSE  3000/tcp
 EXPOSE 3003
 
-COPY . /app
-WORKDIR /app
+COPY . /var/www/html
+WORKDIR /var/www/html
 RUN npm install
 
 CMD npm start

tests/apps/dockerfile/app.json

@@ -11,8 +11,8 @@
   "logo": "https://raw.githubusercontent.com/dokku/dokku/master/docs/assets/dokku.png",
   "scripts": {
     "dokku": {
-      "predeploy": "touch /app/predeploy.test",
-      "postdeploy": "touch /app/postdeploy.test"
+      "predeploy": "mkdir -p /app && touch /app/predeploy.test",
+      "postdeploy": "mkdir -p /app && touch /app/postdeploy.test"
     }
   },
   "env": {