add gcr/gcp docker helper

[dacbd]

similar to the ecr/aws use case.

where gitlab-ci DOCKER_AUTH_CONFIG:

{"credHelpers": { "us-west1-docker.pkg.dev": "gcr"}}

stages:
  - deploy
  - train

deploy_job:
  stage: deploy
  when: always
  image: iterativeai/cml
  script:
    - cml-runner
      --cloud gcp
      --cloud-region us-west
      --cloud-type e2-highcpu-2
      --cloud-permission-set=permission-set-test@cml-gcp-test.iam.gserviceaccount.com,scopes=storage-rw 
      --labels=cml-runner

train_job:
  stage: train
  when: on_success
  needs: [deploy_job]
  image: us-west1-docker.pkg.dev/cml-gcp-test/cml-test/cml:latest
  tags:
    - cml-runner
  script:
    - echo "hello"
    - cml --version

related: https://github.com/iterative/terraform-provider-iterative/issues/627

when published/released closes: https://github.com/iterative/cml/issues/1136

[0x2b3bfa0]

Don't forget to update golden tests.

[tasdomas]

Updated golden tests.