Gitlab CICD stages to build a project, upload docker image to Google Container Registry and update the container of a pre configured Kubernetes Engine Project

.gitlab-ci.yml

services:
  - docker:dind

stages:
  - build
  - package
  - deploy

build:
  stage: build
  image: gradle:slim
  script:
    - ./gradlew build
  artifacts:
    paths:
      - build/libs/*.jar
    expire_in: 1 week
  only:
    - master

docker:
  stage: package
  image: docker:latest
  variables:
    DOCKER_IMAGE_TAG: '<image url>'
  script:
    # Build image
    - docker build --cache-from "${DOCKER_IMAGE_TAG}" -t "${DOCKER_IMAGE_TAG}" .
    # Log in to GCR
    - echo "$SERVICE_ACCOUNT_KEY" | docker login -u _json_key --password-stdin https://eu.gcr.io/cn-2019
    # Push the image
    - docker push ${DOCKER_IMAGE_TAG}
  only:
    - master

deploy:
  stage: deploy
  image: google/cloud-sdk:latest
  variables:
    PROJECT: '<project>'
    DEPLOYMENT_PROJ: '<deployment project>'
  script:
    - echo "$SERVICE_ACCOUNT_KEY" > key.json
    - gcloud auth activate-service-account --key-file=key.json
    - gcloud config set project ${PROJECT}
    - gcloud config set container/cluster ${DEPLOYMENT_PROJ}
    - gcloud config set compute/zone europe-west1-b
    - gcloud container clusters get-credentials ${DEPLOYMENT_PROJ}
    - kubectl scale --replicas=0 deployment ${DEPLOYMENT_PROJ}
    - kubectl scale --replicas=1 deployment ${DEPLOYMENT_PROJ}
  only:
    - master