Suppress clutter from docker-credential-gcloud error messages

[briandealwis]

Follows on #4451

Description

The change in #4451 makes Skaffold's image-pulling follow the approach used with docker pull: first try with credentials, and then without credentials. When pulling from GCR, that first pull may result in an error being emitted from docker-credential-gcloud when there is no user or credentials have been revoked.

This happens surprisingly often — even when users have not run gcloud auth configure-docker — as Skaffold tries using docker-credential-gcloud for *.gcr.io. This is the case with Cloud Code as the Cloud SDK is automatically installed, but users may not yet have performed an action requiring authentication (e.g., to deploy to GCP).

This PR sets CLOUDSDK_CORE_VERBOSITY=none prior to pulling images to suppress error messages emitted from docker-credential-gcloud. These messages are unnecessary as should the image pull fail, the error reported from the registry and shown to the user contains sufficient information to rectify the situation.

Before:

$GCLOUDSDK/bin/skaffold build
Generating tags...
 - skaffold-buildpacks -> skaffold-buildpacks:v1.13.1-49-g98ba2d818
Checking cache...
 - skaffold-buildpacks: Not found. Building
Found [minikube] context, using local docker daemon.
Building [skaffold-buildpacks]...
ERROR: (gcloud.auth.docker-helper) You do not currently have an active account selected.
Please run:

  $ gcloud auth login

to obtain new credentials.

If you have already logged in with a different account:

    $ gcloud config set account ACCOUNT

to select an already authenticated account to use.
v1: Pulling from buildpacks/builder
ab0f59294661: Pulling fs layer 
c2ae8b336f77: Pulling fs layer 

Now:

$ skaffold build
Generating tags...
 - skaffold-buildpacks -> skaffold-buildpacks:v1.13.1-48-g28d305c44
Checking cache...
 - skaffold-buildpacks: Not found. Building
Found [minikube] context, using local docker daemon.
Building [skaffold-buildpacks]...
v1: Pulling from buildpacks/builder
ab0f59294661: Pull complete 
c2ae8b336f77: Pull complete 
3c2cba919283: Pull complete 

Error when pulling from a private repository:

$ skaffold build
Generating tags...
 - skaffold-buildpacks -> skaffold-buildpacks:v1.13.1-50-g456977b80-dirty
Checking cache...
 - skaffold-buildpacks: Not found. Building
Found [minikube] context, using local docker daemon.
Building [skaffold-buildpacks]...
couldn't build "skaffold-buildpacks": failed to fetch builder image 'gcr.io/<private-project>/first:latest': pulling image from repository: Error response from daemon: unauthorized: You don't have the needed permissions to perform this operation, and you may have invalid credentials. To authenticate your request, follow the steps in: https://cloud.google.com/container-registry/docs/advanced-authentication

[codecov]

Codecov Report

Merging #4705 into master will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #4705      +/-   ##
==========================================
- Coverage   73.35%   73.34%   -0.02%     
==========================================
  Files         338      338              
  Lines       13389    13394       +5     
==========================================
+ Hits         9822     9824       +2     
- Misses       2961     2962       +1     
- Partials      606      608       +2     

Impacted Files	Coverage Δ
pkg/skaffold/docker/image.go	75.21% <40.00%> (-0.79%)	⬇️

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[gsquared94]

LGTM!