fix busybox image to 1.28 (issues with `nslookup`).
Changes where done with these commands:
reprec 'image: busybox(?!:)' 'image: busybox:1.28' */docs */examples
reprec -- '--image=busybox(?!:)' '--image=busybox:1.28' */docs */examples
Related issues:
https://github.com/docker-library/busybox/issues/48
https://github.com/kubernetes/kubernetes/issues/66924