x/build: make helper binary wrapper around docker push & kubectl

[bradfitz]

Our Makefiles in x/build make it too easy to screw up, since they assume hidden gcloud & kubectl & docker state.

I just spent hours debugging a docker push auth issue (finally resolved with help of gcr.io team), and then I screwed up a deploy due to:

deploy-prod: push-prod
        kubectl set image deployment/coordinator-deployment coordinator=$(IMAGE_PROD):$(VERSION)
deploy-staging: push-staging
        kubectl set image deployment/coordinator-deployment coordinator=$(IMAGE_STAGING):$(VERSION)

Note that deploy-staging rule doesn't validate that my kubectl is in the right cluster/context.

I'm going to fix this with a helper Go program around all these such operations that can additionally verify things are correct.

Likewise with the various gcloud commands.

/cc @andybons @FiloSottile @bcmills

[gopherbot]

Change https://golang.org/cl/112035 mentions this issue: cmd/xb: add new safety command to double-check state of deployment commands

[bradfitz]

I did more in https://go-review.googlesource.com/c/build/+/113135

[bradfitz]

Sufficiently done. We can add more over time.