Ensure DOKU_INC is always right using an auto_prepend_file

This is basically the same fix as bitnami/containers#12535

Plugins or template may have their own entry points. Eg. the sprintdoc
template's svg.php

Those entrypoints usually start with something like:

```
if(!defined('DOKU_INC')) define('DOKU_INC', __DIR__ . '/../../../');
require_once(DOKU_INC . 'inc/init.php');
```

Since plugins are installed in the volume, __DIR__ will point to the
wrong location. By predefining DOKU_INC before any other code runs, this
can be avoided.

root/usr/local/etc/php/conf.d/dokuwiki.ini

@@ -3,3 +3,4 @@ post_max_size = ${PHP_UPLOADLIMIT}
 upload_max_filesize = ${PHP_UPLOADLIMIT}
 memory_limit = ${PHP_MEMORYLIMIT}
 date.timezone = ${PHP_TIMEZONE}
+auto_prepend_file = /var/www/html/.autoprepend.php

root/var/www/html/.autoprepend.php

@@ -0,0 +1,7 @@
+<?php
+/**
+ * This file is automatically loaded by PHP before any other code. This ensures that DOKU_INC is
+ * is always pointing to the correct location within the container, even when the entry point is
+ * a plugin.
+ */
+if (!defined('DOKU_INC')) define('DOKU_INC', __DIR__ . '/');

root/var/www/html/health.php

@@ -1,7 +1,7 @@
 <?php
 
 define('DOKU_UNITTEST', 1);
-define('DOKU_INC', __DIR__ . '/');
+if(!defined('DOKU_INC')) define('DOKU_INC', __DIR__ . '/');
 
 header('Content-Type: text/plain; charset=utf-8');
 try {