Changes:
- install.sh: user-template-next is now only built if USER_TEMPLATE_IMAGE=user-template-next:latest is set in .env
* Defaults to user-service-template (nginx) to save 4-5 minutes per install
* Dynamic build step counting based on configured templates
* Shows helpful message when template is skipped
* Build numbering adapts automatically ([1/3] vs [1/4])
- user-template-next/Dockerfile: Optimize build performance
* Pin Node version to 20.11-alpine for reproducibility
* Use npm ci instead of npm install for faster, reproducible builds
* Separate package.json copy for better layer caching
* Add --prefer-offline and --no-audit flags to npm ci
* Clean npm cache to reduce image size
* Add clear comments for multi-stage build steps
Impact:
- Default installations: 2-3 minutes faster
- Reduced build time for Next.js template (when enabled) via layer caching
- Better reproducibility and predictable builds
- All build logs now written to spawner-install.log
- Added clear section headers for each build
- Replaced separate temp log files with single log file
- Added log file path hint at end of installation
