Google Cloud Build: K8s application rolling update deployment on repository code push

cloudbuild.yaml

steps:
# slack deployment status
- name: 'gcr.io/cloud-builders/curl'
  args: [ '-X', 'POST', '-H', 'Content-type: application/json', '--data', 
          '{"text":"`gcr.io/$PROJECT_ID/$_DEPLOYMENT:$BRANCH_NAME$TAG_NAME` build started!"}', '${_WEBHOOK_URL}' ]

# build docker image
- name: 'gcr.io/cloud-builders/docker'
  args: [ 'build', '--build-arg', 'TOKEN=${_OAUTH_TOKEN}', 
          '-t', 'gcr.io/$PROJECT_ID/$_DEPLOYMENT:$BRANCH_NAME$TAG_NAME', '.' ]

# push docker image to the repository
- name: 'gcr.io/cloud-builders/docker'
  args: [ 'push', 'gcr.io/$PROJECT_ID/$_DEPLOYMENT:$BRANCH_NAME$TAG_NAME' ]

# patch kubernetes deployment to trigger rolling update
- name: 'gcr.io/cloud-builders/kubectl'
  args: [ 'patch', 'deployment', '${_DEPLOYMENT}', '-n', '${_NAMESPACE}', '-p',
          '{"spec": {"template": {"metadata": {"annotations": {"build":"$BUILD_ID"}}, "spec": {"initContainers": [{"name": "app","image": "gcr.io/$PROJECT_ID/$_DEPLOYMENT:$BRANCH_NAME$TAG_NAME"}]}}}}' ]
  env:
  # required K8s cluster env variables
  - 'CLOUDSDK_COMPUTE_ZONE=${_K8S_ZONE}'
  - 'CLOUDSDK_CONTAINER_CLUSTER=${_K8S_CLUSTER}'

# check rolling deployment status
- name: 'gcr.io/cloud-builders/kubectl'
  args: [ '-n', '${_NAMESPACE}', 'rollout', 'status', 'deploy/${_DEPLOYMENT}' ]
  env:
  # required K8s cluster env variables
  - 'CLOUDSDK_COMPUTE_ZONE=${_K8S_ZONE}'
  - 'CLOUDSDK_CONTAINER_CLUSTER=${_K8S_CLUSTER}'

# slack deployment status
- name: 'gcr.io/cloud-builders/curl'
  args: [ '-X', 'POST', '-H', 'Content-type: application/json', '--data', 
          '{"text":"${_DEPLOYMENT} deployment completed!"}', '${_WEBHOOK_URL}' ]

# store artifact
images: ['gcr.io/$PROJECT_ID/$_DEPLOYMENT:$BRANCH_NAME$TAG_NAME']

# arguments
substitutions:
    _K8S_ZONE:      us-central1-a   # K8s zone
    _K8S_CLUSTER:   cluster_name    # K8s cluster name
    _NAMESPACE:     default         # K8s namespace
    _DEPLOYMENT:    deployment_name # K8s deployment name
    _WEBHOOK_URL:   slack_notification_webhook  # slack notifications
    _OAUTH_TOKEN:   composer_github_oauth_token # access to private code repos