fix: add missing build steps and update Node in Dockerfile

[garythebat]

Summary

Re-targeted from #476 to rel-next as requested.

• Update base image from node:18-slim to node:22-slim (matching package.json engines requirement)
• Add git to apt-get install (needed by npm dependencies)
• Add webpack-ext step (builds external bundles required by esbuild)
• Add mkdir -p src/pack (prevents race condition on clean builds)
• Add pack-prod step (builds application bundles)
• Reorder: build steps run before gs-app-server global install

Without these changes, the container starts but serves an incomplete app that never finishes loading - the JS bundles in src/pack/ are never built.

Test environment

• ARM64: Raspberry Pi 4 (8GB), Pi OS Bookworm, Docker 29.4.0, Node 22
• x86_64: Ubuntu 24.04 (Proxmox LXC), Docker 28.0.4, Node 22
• Both: clean clone, docker compose build, verified /kiri/ loads and slices

Test plan

• docker compose build completes without error
• Container starts and /kiri/ returns 200
• /lib/pack/kiri-main.js returns 200 with non-empty content
• Slicer UI loads fully in browser
• Import STL and slice produces gcode