feat: Improve FrankenPHP container

[tweis]

Adds cron support to the FrankenPHP images and cleans up prometheus/healthcheck config, plus keeps build-container.sh and the CI workflow in sync with the Dockerfile targets.

• Add frankenphp-cron / frankenphp-cron-dev targets — new stages that install a crontab from $CRONTAB_PATH and run cron -f, mirroring the existing fpm/cli/dev split. Symlinks /usr/local/bin/php to /usr/bin/php since cron's minimal PATH doesn't include /usr/local/bin.
• Fix healthcheck scoping — HEALTHCHECK NONE on the shared frankenphp-base stage, with the real healthcheck only applied to the frankenphp/frankenphp-dev leaf targets (previously cli/cron images could inherit an inapplicable healthcheck).
• Fix prometheus metrics port — corrected port/label mismatch and added EXPOSE 2020 on the runtime targets.
• Fix build context leak — frankenphp no longer COPYs the Docker build context into /var/www/; app code is mounted/copied separately.
• Rename app-frankenphp-base → frankenphp-base for consistency with the other target names.
• Add pdo_pgsql PHP extension.
• CI/build script: added GitHub Actions build-and-push steps for frankenphp-cron/frankenphp-cron-dev, and updated build-container.sh to match (same build args/flags as the workflow, added missing 8.0 fpm-dev/cli-dev lines, added 8.5 and node:24 targets).