[ci/on-merge] Trigger container image build (#148864)

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.

Tests passing -> image build:
https://buildkite.com/elastic/kibana-on-merge/builds/25854
Tests failing -> no image build:
https://buildkite.com/elastic/kibana-on-merge/builds/25849

.buildkite/pipelines/on_merge.yml

@@ -149,3 +149,12 @@ steps:
     timeout_in_minutes: 10
     agents:
       queue: kibana-default
+
+  - wait
+
+  - trigger: kibana-artifacts-container-image
+    label: Trigger container image build
+    async: true
+    build:
+      commit: "${BUILDKITE_COMMIT}"
+      branch: "${BUILDKITE_BRANCH}"