Allow use custom registry username
This allow to support other private registries. For example, to use gcr.io, define the following variables in .gitlab-ci.yaml:
- CI_REGISTRY_USER: _json_key
- CI_REGISTRY_IMAGE: gcr.io/GCR_PROJECT_NAME/APP_NAME
- CI_REGISTRY: https://gcr.io
And define CI_BUILD_TOKEN in project variable setting using gcloud service account token.