fix: respect the build-dir in app-json extraction

Because the app-json extraction happens before the build-dir is set, it doesn't end up respecting the value, and thus the user might get the wrong app.json file.

This takes advantage of path.Join()'s default of ignoring empty paths.

docs/development/plugin-triggers.md

@@ -221,6 +221,22 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 # TODO
 ```
 
+### `builder-get-property`
+
+- Description: Return the value for an app's builder property
+- Invoked by:
+- Arguments: `$APP $KEY`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+APP="$1"; PROPERTY="$2"
+
+# TODO
+```
+
 ### `builder-image-is-cnb`
 
 - Description: Checks if an image is cnb-based