fix(docker): ARG CACHEBUST to invalidate layer cache for cron install

Docker cached the apt-get layer from before cron was added. ARG before
RUN forces layer rebuild.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

 FROM php:8.2-apache-bookworm
 
-# Cache-bust: 2026-07-03 (force rebuild for cron package)
 # ── System dependencies ──
+ARG CACHEBUST=2026-07-03
 RUN apt-get update && apt-get install -y \
     libpng-dev \
     libjpeg-dev \