Set commit msg when updating gitops repo to publish new image

This update sets the commit message to push to the gitops repository when a new Kibana image is published.

Also to align with the other services the IMAGE_TAG is prefixed by `git-` and use a 12 characters short commit

.buildkite/scripts/steps/artifacts/docker_image.sh

@@ -6,8 +6,8 @@ set -euo pipefail
 
 source .buildkite/scripts/steps/artifacts/env.sh
 
-GIT_ABBREV_COMMIT=${BUILDKITE_COMMIT:0:7}
-KIBANA_IMAGE="docker.elastic.co/kibana-ci/kibana:$GIT_ABBREV_COMMIT"
+GIT_ABBREV_COMMIT=${BUILDKITE_COMMIT:0:12}
+KIBANA_IMAGE="docker.elastic.co/kibana-ci/kibana:git-$GIT_ABBREV_COMMIT"
 
 echo "--- Verify manifest does not already exist"
 echo "$KIBANA_DOCKER_PASSWORD" | docker login -u "$KIBANA_DOCKER_USERNAME" --password-stdin docker.elastic.co
@@ -82,16 +82,17 @@ if [[ "$BUILDKITE_BRANCH" == "$KIBANA_BASE_BRANCH" ]]; then
   cat << EOF | buildkite-agent pipeline upload
 steps:
   - trigger: k8s-gitops-update-image-tag
-    label: ":argo: Update image tag for deployment-api"
+    label: ":argo: Update image tag for Kibana"
     branches: main
     build:
       env:
         MODE: sed
         TARGET_FILE: kibana-controller.yaml
-        IMAGE_TAG: "$GIT_ABBREV_COMMIT"
+        IMAGE_TAG: "git-$GIT_ABBREV_COMMIT"
         SERVICE: kibana-controller
         NAMESPACE: kibana-ci
         IMAGE_NAME: kibana
+        COMMIT_MESSAGE: "gitops: update kibana tag to elastic/kibana@$BUILDKITE_COMMIT"
 EOF
 
 else