feat: add Makefile targets for project-clone and project-backup images

[akurinnoy]

Summary

• Adds multi-arch build and push Makefile targets for project-clone and project-backup images, matching the existing controller image pattern
• Previously these images required manual podman build/docker build commands that produced single-arch images only
• All new targets support Docker (with buildx) and Podman, producing multi-arch (amd64 + arm64) images with proper manifest lists

New targets:

• docker-project-clone — build and push project-clone
• docker-project-backup — build and push project-backup
• docker-all — build and push all three images (controller + project-clone + project-backup)

Updated docs: docs/release/README.md prerelease testing section now uses make docker-all

Test plan

• make -n docker-all dry-run produces correct build/push commands for all 3 images
• make help lists all new targets
• Build and push with Docker: make docker-all DWO_IMG=... PROJECT_CLONE_IMG=... PROJECT_BACKUP_IMG=...
• Build and push with Podman: DOCKER=podman make docker-all ...
• Push confirmation prompt appears when targeting default quay.io/devfile/*:next images
• Push confirmation is skipped with INITIATOR=CI

🤖 Generated with Claude Code

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This pull-request has been approved by: akurinnoy
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