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/worker.js

@@ -0,0 +1,6 @@
+function worker() {
+  console.log('sleeping for 60 seconds');
+  setTimeout(worker, 60 * 1000);
+}
+
+worker();