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

docs/advanced-usage/deployment-tasks.md

@@ -13,7 +13,8 @@ Common use cases include:
 
 Dokku accomplishes this by using an `app.json` file. The format in use is similar to format of Heroku's [app.json](https://devcenter.heroku.com/articles/app-json-schema).
 However, dokku currently only supports the nodes `scripts.dokku.predeploy` and `scripts.dokku.postdeploy`.
-Simply place an `app.json` file in the root of your repository or for dockerfile apps, `ADD` it to /app.
+For buildpack apps, simply place an `app.json` file in the root of your repository.
+For dockerfile apps, place `app.json` in the configured `WORKDIR` directory; otherwise dokku defaults to the buildpack app behavior of looking in `/app`.
 >NOTE: postdeploy changes are *NOT* committed to the app image.
 
 ### Example app.json

plugins/00_dokku-standard/exec-app-json-scripts

@@ -10,7 +10,7 @@ get_phase_script() {
   local APP_JSON_FILE="$GET_PHASE_SCRIPT_TMP_WORK_DIR/app.json"
   trap 'rm -rf "$GET_PHASE_SCRIPT_TMP_WORK_DIR" > /dev/null' RETURN INT TERM
 
-  copy_from_image "$IMAGE" "/app/app.json" "$GET_PHASE_SCRIPT_TMP_WORK_DIR" 2>/dev/null || true
+  copy_from_image "$IMAGE" "app.json" "$GET_PHASE_SCRIPT_TMP_WORK_DIR" 2>/dev/null || true
 
   if [[ -f "$APP_JSON_FILE" ]];then
     local VALUE=$(get_json_value "scripts.dokku.${PHASE_SCRIPT_KEY}" < "$APP_JSON_FILE")

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": {