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/check_deploy

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -e
+output="$(curl -s -S "$1")"
+echo "$output"
+test "$output" == "nodejs/express"