Fix first element being removed from $PATHEXT

See https://github.com/xp-framework/core/pull/279/files#r709533545

Author: thekid

src/main/php/xp/lambda/Docker.class.php

@@ -17,7 +17,7 @@ private function resolve($commands) {
     clearstatcache();
 
     // PATHEXT is in form ".{EXT}[;.{EXT}[;...]]"
-    $extensions= [''] + explode(PATH_SEPARATOR, getenv('PATHEXT'));
+    $extensions= array_merge([''], explode(PATH_SEPARATOR, getenv('PATHEXT')));
     $paths= $paths= explode(PATH_SEPARATOR, getenv('PATH'));
     foreach ($commands as $command) {