Add BuildKit documentation as secure alternative to Kaniko

Lysanne Pintorequested to merge
docs-lyspin-536071-using-buildx-to-build-docker-images into master

What does this MR do and why?

Adds docs for building container images using BuildKit:

  • Positions BuildKit rootless as the Kaniko replacement
  • Provides migration guidance
  • Offers alternatives for users who don't need maximum security

🧪 Review app

https://docs.gitlab.com/upstream-review-mr-ee-192429/ci/docker/using_buildkit/

📚 References

Edited by Lysanne Pinto

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