Add hack/tools.go and "prettify" Dockerfiles

[justaugustus]

I've made some cosmetic changes to the Dockerfiles in build/ and images/. Now that kubernetes/test-infra#14735 and kubernetes/test-infra#14711 have landed, this should trigger pushes for the container images to our staging GCP project.

This PR also adds a hack/tools.go, which includes an empty import for the k8s.io/release/build/debs submodule (to ensure we pull in its' dependencies when someone go gets k/release).

/assign @fejta @Katharine @dims @tpepper
cc: @kubernetes/release-engineering
/priority important-soon
/kind cleanup
/milestone v1.17

[k8s-ci-robot]

@justaugustus: Adding label: do-not-merge/blocked-paths because PR changes a protected file.

Reasons for blocking this PR:

[Changes to certain release tools can affect our ability to test, build, and release Kubernetes. This PR must be explicitly approved by SIG Release repo admins.]

• build/debs/Dockerfile

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[fejta]

/lgtm
/hold
(so you can merge when you want)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fejta, justaugustus

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[justaugustus]

Thanks @fejta!
/hold cancel
/honk

[k8s-ci-robot]

@justaugustus:

In response to this:

Thanks @fejta!
/hold cancel
/honk

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