ci: add gcloud docker configure command

Created by: ggilmore

After https://github.com/sourcegraph/sourcegraph/pull/5323, CI is currently failing with this error:

authorized: 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

After reading https://cloud.google.com/container-registry/docs/advanced-authentication#gcloud_as_a_docker_credential_helper, I believe gcloud auth configure-docker is what we need run in order to grant the credentials.