Switch to official php:8.5.8-fpm-trixie image, fix extension/FPM build issues

Rebuild on the official PHP image instead of Sury's repo on bare Debian.
Fixes surfaced along the way: missing pkg-config/libonig-dev broke the
zip/mbstring extension builds, purging libzip-dev also auto-removed the
libzip5 runtime lib the compiled zip.so needs, phpredis needed bumping to
6.3.0 for PHP 8.5 header compatibility, and the php-fpm.conf listen
directive sed didn't match the commented-out default line so FPM never
created its unix socket, causing 503s from Apache's proxy_fcgi handler.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -9,7 +9,8 @@ It provides a streamlined stack featuring Apache, the latest PHP version, Redis,
 
 ## Features
 
-- **PHP Version**: 8.5.3
+- **Base**: official `php:8.5.8-fpm-trixie` image (Debian Trixie)
+- **PHP Version**: 8.5.8 (pinned)
 - **Apache Web Server**: Configured to run PHP applications
 - **Support for**:
   - `remoteip` (reverse proxy)
@@ -33,9 +34,9 @@ cd corxn
 
 ```bash
 sudo su
-docker buildx build --no-cache -t 4lights/corxn:8.5.3 -t 4lights/corxn:latest --load .
+docker buildx build --no-cache -t 4lights/corxn:8.5.8 -t 4lights/corxn:latest --load .
 docker login -u <username>
-docker push 4lights/corxn:8.5.3
+docker push 4lights/corxn:8.5.8
 docker push 4lights/corxn:latest
 ```
 # Testing