gcloud Docker Login by Default

[RyanTimoney]

Currently, Docker needs to be logged into the Google Container Registry by code exterior to Servicer. Example:

docker_example:
    provider: gcloud
    service_type: docker_image
    steps:
      build:
        config:
          steps:
            - type: build
              args:
                image: demo-image
                dockerfile: Dockerfile
      deploy:
        config:
          registry_path: my-gcp-project/this-project
          steps:
            - type: run
              args: 
                command: docker login -u _json_key -p "$(cat /home/someuser/my-gcr-keyfile.json)" https://us.gcr.io/my-gcp-project
            - type: push
              args:
                image: demo-image
                tags:
                  - latest

The push will fail without the login command. This should be baked into the servicer/builtin/service_adapters/gcloud/docker_image.py or similar.