feat: make default NGINX Plus Dockerfile use buildkit

[alessfg]

Proposed changes

This PR refactors the NGINX Plus Dockerfile makes the Buildkit Dockerfile the default NGINX Plus build given that both Buildkit is now the default Docker engine and that using Buildkit is a better build option security (and in doing so remove the current existing default Plus Dockerfile).

Part of this original commit has been split into a different PR #274.

Checklist

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• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (e.g. README.md).