Default to gcloud auth

[dgageot]

Make sure that gcloud doesn't need to be configured as a helper to be used
to push to/pull from *.gcr.io

This used to be the case but using GetAllAuthConfigs() over GetAuthConfig(registry string) broke this behaviour.

Also use the gcloud auth code from go-containerregistry

[codecov]

Codecov Report

Merging #3282 into master will decrease coverage by 0.01%.
The diff coverage is 64.28%.

Impacted Files	Coverage Δ
pkg/skaffold/gcp/auth.go	28.12% <100%> (-9.72%)	⬇️
pkg/skaffold/docker/auth.go	56.14% <50%> (+0.37%)	⬆️
pkg/skaffold/server/server.go	58.57% <0%> (ø)	⬆️

[balopat]

This is failing with gcloud auth errors on integration tests. PTAL @dgageot

[dgageot]

@balopat I cut the PR in two pieces. The other part needs some more work. This part passes the tests

[briandealwis]

So one positive from the previous code was that the credential-helpers were only configured if the registry was a gcr.io repo. And we logged that we were auto-configuring it (since it wasn't in their docker config.json).

Should we also checking for docker-credential-gcr too?

[briandealwis]

This means a broken config.json will cause skaffold to abort. Maybe we should log an error, but continue on?

[dgageot]

1/ docker-credential-gcloud should be favored. See https://github.com/GoogleCloudPlatform/docker-credential-gcr "For normal development setups, users are encouraged to use gcloud auth configure-docker, instead"
2/ In the docker build code, we ignore the error, like the Docker CLI does it.
3/ We can't just configure it for gcr.io when GetAllAuthConfigs() is used. When GetAuthConfig is used, configuring the helper for *.gcr.io has the exact same effect

[nkubala]

I won't lie, I'm always a bit hesitant with auth changes, but everything looks fine to my eye. let's merge this and then make sure and monitor the next release to make sure we're not breaking existing users.