feat: disable scaling if app contains DOKKU_SCALE file

A DOKKU_SCALE file will always be used for managing the scale count for an app if it exists in a repository. As such, ps:scale usage should be disabled in those cases. This commit makes that more apparent.

Also document how to build a specific core plugin for testing purposes.

Closes #3544

tests/apps/dockerfile-dokku-scale/CHECKS

@@ -0,0 +1,5 @@
+WAIT=2 # wait 2 seconds
+TIMEOUT=5 # set timeout to 5 seconds
+ATTEMPTS=2 # try 2 times
+
+/ nodejs/express

tests/apps/dockerfile-dokku-scale/DOKKU_SCALE

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+web=1

tests/apps/dockerfile-dokku-scale/Dockerfile

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+FROM node:4
+
+EXPOSE 3001/udp
+EXPOSE  3000/tcp
+EXPOSE 3003
+
+COPY . /app
+WORKDIR /app
+RUN npm install
+
+CMD npm start

tests/apps/dockerfile-dokku-scale/Procfile

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+###############################
+#         DEVELOPMENT         #
+###############################
+
+# Procfile for development using the new threaded worker (scheduler, twitter stream and delayed job)
+cron: node worker.js
+web: node web.js
+worker: node worker.js

tests/apps/dockerfile-dokku-scale/check_deploy

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+#!/usr/bin/env bash
+set -e
+output="$(curl -s -S "$1")"
+echo "$output"
+test "$output" == "nodejs/express"

tests/apps/dockerfile-dokku-scale/package.json

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+{
+  "name": "node-example",
+  "version": "0.0.1",
+  "dependencies": {
+    "express": "2.5.x"
+  },
+  "engines": {
+    "node": "4.2.x"
+  },
+  "scripts": {
+    "start": "false"
+  }
+}

tests/apps/dockerfile-dokku-scale/web.js

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+var express = require('express');
+
+var app = express.createServer(express.logger());
+
+app.get('/', function(request, response) {
+  response.send('nodejs/express');
+});
+
+var port = process.env.PORT || 5000;
+app.listen(port, function() {
+  console.log("Listening on " + port);
+});

tests/apps/dockerfile-dokku-scale/worker.js

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+function worker() {
+  console.log('sleeping for 60 seconds');
+  setTimeout(worker, 60 * 1000);
+}
+
+worker();