refactor: inline some rules_docker 'skylib' helpers

[alexeagle]

Note: I think it might be better to introduce a dep on aspect_bazel_lib instead, which is a very-well maintained utility library that's a superset of bazel_skylib, and evidently rules_docker wished for such a thing as well since it had this folder.

Baby steps towards #723 I'd like to start small and gauge interest from rules_k8s maintainers before spending too much time here.

[k8s-ci-robot]

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