Add LinuxKit configurations for developer and server-php images:
- developer/linuxkit.yml: Full dev environment with Docker-in-LinuxKit
- server-php/linuxkit.yml: Nginx + PHP-FPM production server
Update CI workflow to build LinuxKit images using `core build --type linuxkit`
instead of raw linuxkit CLI commands for consistency across the ecosystem.
Builds produce qcow2 and ISO formats for both amd64 and arm64 architectures.
Release artifacts are uploaded to GitHub Releases on version tags.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
