Cloud Build for Firebase NodeJS app with start/complete/fail slack notifications

cloudbuild.yaml

steps:
  # build docker image
  - name: "gcr.io/cloud-builders/docker"
    entrypoint: "bash"
    args:
    - "-c"
    - |
      curl -X POST -H "Content-type: application/json" --data \
      '{"text":"`gcr.io/$PROJECT_ID/$REPO_NAME:$BRANCH_NAME$TAG_NAME` build ID `$BUILD_ID` started!"}' $_WEBHOOK_URL
      docker build \
      --build-arg PROJECT_ID=$PROJECT_ID \
      --build-arg NPM_TOKEN=$_NPM_TOKEN \
      --build-arg FIREBASE_TOKEN=$_FIREBASE_TOKEN \
      -t gcr.io/$PROJECT_ID/$REPO_NAME:$BRANCH_NAME$TAG_NAME . \
      && curl -X POST -H "Content-type: application/json" --data \
      '{"text":"`gcr.io/$PROJECT_ID/$REPO_NAME:$BRANCH_NAME$TAG_NAME` build ID `$BUILD_ID` completed!"}' $_WEBHOOK_URL \
      || curl -X POST -H "Content-type: application/json" --data \
      '{"text":"`gcr.io/$PROJECT_ID/$REPO_NAME:$BRANCH_NAME$TAG_NAME` build ID `$BUILD_ID` failed!"}' $_WEBHOOK_URL
    timeout: 1500s

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

timeout: 1500s

# store artifact
images: ["gcr.io/$PROJECT_ID/$REPO_NAME:$BRANCH_NAME$TAG_NAME"]

# user high CPU machine for the build
# options:
#  machineType: "N1_HIGHCPU_8"

# arguments
substitutions:
  _NPM_TOKEN: default_npm_access_token
  _FIREBASE_TOKEN: default_firebase_deployment_token
  _WEBHOOK_URL: default_slack_notification_webhook