add helm chart build and publish pipeline

.gitlab-ci.yml

 ---
+image:
+  name: alpine/helm:3.7.1
+  entrypoint: ["/bin/sh", "-c"]
 
 # A Gitlab CI job template for building docker images using kaniko.
 #
@@ -25,9 +28,9 @@
 
 stages:
   - build
-
-default:
-  image: "${CI_REGISTRY_IMAGE}/${KANIKO_BUILD_IMAGENAME}:${CI_COMMIT_REF_SLUG}"
+  - lint-helm-chart
+  - package-helm-chart
+  - release-helm-chart
 
 build-wp-cli:
   stage: build
@@ -60,3 +63,49 @@ build-wp:
       when: on_success
     - if: '$CI_COMMIT_TAG'
       when: on_success
+
+# Only package a new chart once the version number has been updated
+.chart_release_rules:
+  rules:
+    - changes:
+      - Chart.yaml
+
+lint-helm:
+  stage: lint-helm-chart
+  script:
+    - helm dep update
+    - helm lint .
+  artifacts:
+    paths:
+      - 'charts/**'
+    expire_in: 1 week
+    # Even if lint fails, upload the charts/ folder as artifact
+    when: always
+  allow_failure: true
+  rules:
+    - changes:
+      - '*.yaml'
+      - templates/*.yaml
+
+package-chart:
+  stage: package-helm-chart
+  script:
+    - helm package .
+  artifacts:
+    paths:
+      - wordpress-*
+    expire_in: 1 week
+  extends:
+    - .chart_release_rules
+
+# Push helm chart. Charts on the `main` branch are pushed to `stable`, others
+# are pushed to the `unstable` channel.
+release-helm:
+  image: "rancher/curlimages-curl:7.73.0"
+  stage: release-helm-chart
+  script:
+    - if [ "$CI_COMMIT_BRANCH" == "main" ]; then export HELM_CHANNEL='stable'; else export HELM_CHANNEL='unstable'; fi
+    - export CHART_FILE=$(ls wordpress-*.tgz)
+    - curl --request POST --user gitlab-ci-token:$CI_JOB_TOKEN --form "chart=@${CHART_FILE}" "${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/packages/helm/api/${HELM_CHANNEL}/charts"
+  extends:
+    - .chart_release_rules