Add Drone deployment

This change adds Drone configuration to trigger a new deployment to the
appropriate Kubernetes namespace.

.drone.yml

@@ -0,0 +1,69 @@
+---
+
+kind: pipeline
+type: kubernetes
+name: helm lint
+
+platform:
+  os: linux
+  arch: amd64
+
+trigger:
+  event:
+    - pull_request
+  branch:
+    - main
+
+steps:
+  - name: lint-helm-chart
+    image: pelotech/drone-helm3
+    settings:
+      mode: lint
+      chart: helm
+      values_files: helm/values.yaml
+      values:
+        - app.image.version=$${DRONE_COMMIT_SHA}
+
+---
+kind: pipeline
+type: kubernetes
+name: deployment
+
+platform:
+  os: linux
+  arch: amd64
+
+trigger:
+  event:
+    - push
+  branch:
+    - main
+
+steps:
+  - name: Wait for image tag
+    image: alpine/curl:8.2.1
+    commands:
+    - |
+       while ! curl -s -L 'https://quay.io/api/v1/repository/ukhomeofficedigital/engineering-guidance-and-standards/tag?specificTag=${DRONE_COMMIT_SHA}' | grep -q '${DRONE_COMMIT_SHA}' ; do
+          echo "Tag not present, waiting 10 seconds"
+          sleep 10
+        done
+
+  - name: Deployment
+    image: pelotech/drone-helm3
+    settings:
+      chart: helm
+      mode: upgrade
+      namespace: sas-prod
+      release: engineering-guidance-and-standards
+      kube_certificate:
+        from_secret: kube_cert_acp_prod
+      kube_api_server: https://kube-api-prod.prod.acp.homeoffice.gov.uk
+      kube_token:
+        from_secret: kube_token_acp_prod
+      values_files: helm/values.yaml
+      wait_for_upgrade: true
+      timeout: 10m
+      history_max: 3
+      values:
+        - app.image.version=$${DRONE_COMMIT_SHA}