refactor: Unify build system for regular and rootless images

- Create build-unified.py that handles both regular and rootless builds
- Convert build.py and build-rootless.py to wrapper scripts for backwards compatibility
- Update CI workflow to use unified build command
- Add BUILD_MIGRATION.md documentation
- Eliminate code duplication between build scripts
- Support flexible build options: --rootless, --both, --only-stable-latest

This maintains all existing functionality while providing a cleaner, more maintainable build system.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/docker-build.yml

@@ -20,18 +20,10 @@ jobs:
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
 
-      - name: build and push
+      - name: build and push all images
         if: ${{ env.DOCKER_USERNAME != '' && env.DOCKER_PASSWORD != '' }}
         env:
           DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
           DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
         run: |
-          ./build.py --push-tags --multiarch
-
-      - name: build and push rootless images
-        if: ${{ env.DOCKER_USERNAME != '' && env.DOCKER_PASSWORD != '' }}
-        env:
-          DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
-          DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
-        run: |
-          ./build-rootless.py --push-tags --multiarch
+          ./build-unified.py --push-tags --multiarch --both