fix(docker): install patched gzip package

[tomatotomata]

Summary

This is the separate app-image package fix requested on #55. The scan found gzip 1.13-1; with Debian Trixie’s current package index, explicitly installing gzip upgrades it to 1.13-1+deb13u1. This leaves the Trivy policy and #55 workflow unchanged.

Validation

• Built .docker/app/Dockerfile with NEXTCLOUD_VERSION=34-fpm for linux/amd64 and linux/arm64.
• The package selected was gzip 1.13-1+deb13u1 on both architectures.
• Trivy 0.74.0 scans using the existing HIGH/CRITICAL and fixed-vulnerability policy completed with zero findings on both app images.
• git diff --check passed.