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/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "node-example",
+  "version": "0.0.1",
+  "dependencies": {
+    "express": "2.5.x"
+  },
+  "engines": {
+    "node": "4.2.x"
+  },
+  "scripts": {
+    "start": "false"
+  }
+}