[ci/on-merge] Trigger container image build (#149350)

Reopens #148864 to trigger via REST instead of yaml. The previous
implementation did not support commit triggered builds.

This conditionally adds a pipeline trigger to
`kibana-artifacts-container-image` at the end of the on-merge pipeline
when tests are passing. The triggered pipeline will build (and
eventually push) our default docker images.

.buildkite/pipelines/on_merge.yml

@@ -149,3 +149,11 @@ steps:
     timeout_in_minutes: 10
     agents:
       queue: kibana-default
+
+  - wait
+
+  - command: '.buildkite/scripts/steps/artifacts/docker_image_trigger.sh'
+    label: Trigger container image build
+    timeout_in_minutes: 10
+    agents:
+      queue: 'kibana-default'

.buildkite/scripts/steps/artifacts/docker_image_trigger.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+set -euo pipefail
+
+ts-node .buildkite/scripts/steps/trigger_pipeline.ts kibana-artifacts-container-image "$BUILDKITE_BRANCH" "$BUILDKITE_COMMIT"