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

docs/development/testing.md

@@ -32,6 +32,9 @@ While we do provide official packages for a variety of platforms, as our test su
   # run linter & update vagrant Dokku install from local git clone
   make lint copyfiles
 
+  # build a specific plugin
+  make go-build-plugin copyplugin PLUGIN_NAME=apps
+
   # execute all bats tests
   make unit-tests