chore: upgrade image tags, harden Dockerfiles, and fix build scripts (#67)

- Upgrade Node.js 25.4 → 26.1, PHP 8.5.3 → 8.5.6, Rust 1.93 → 1.95.0
- Update vulnerable Java deps: commons-compress 1.25.0 → 1.27.1, pdfbox 3.0.1 → 3.0.4
- Bump cryptography ≥44.0.1 (addresses CVE-2024-12797)
- Add SHELL directive and apt cleanup to rust.Dockerfile (security hardening)
- Add PLATFORM variable to build-images.sh for flexible local builds
- Improve .dockerignore: remove duplicates and add missing exclusions
- Fix validate-multiarch-builds.sh: correct nonexistent shell.Dockerfile references
  to the actual changed Dockerfiles (nodejs, php, rust); amd64-only builds