fix(server-php): add empty patch directory for builder stage

The builder stage copies patch/ to allow overriding vendor files.
For the base image this is empty with a README explaining usage.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server-php/patch/README.md

@@ -0,0 +1,20 @@
+# Patch Directory
+
+This directory contains files that override vendor packages after composer install.
+
+## Usage
+
+Place files here that should overwrite files in `vendor/` or elsewhere after dependencies are installed. The entire contents of this directory are copied over the application root.
+
+## Example
+
+To patch a vendor file:
+```
+patch/vendor/some-package/src/File.php
+```
+
+This will overwrite `vendor/some-package/src/File.php` in the built image.
+
+## Empty by Default
+
+For the base image, this directory is empty. Applications using this image should mount or copy their own patches.