CI: Build stackhpc-kayobe-config image on push to release branches

This provides us with an image for each release branch that we can use
in upgrade testing.

Author: markgoddard

.github/workflows/stackhpc-build-kayobe-image.yml

@@ -5,6 +5,11 @@
 name: Build kayobe image
 
 on:
+  push:
+    branches:
+      # NOTE(upgrade): Reference only the current release branch here.
+      - stackhpc/yoga
+
   workflow_call:
     inputs:
       http_proxy:
@@ -32,7 +37,7 @@ env:
 jobs:
   build-kayobe-image:
     name: Build kayobe image
-    if: inputs.if
+    if: inputs.if || github.repository == 'stackhpc/stackhpc-kayobe-config' && github.event_name == 'push'
     runs-on: ubuntu-20.04
     permissions:
       contents: read

releasenotes/notes/kayobe-image-f1227adacb28f40b.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    StackHPC Kayobe Configuration container images for CI/CD with `Kayobe
+    Automation <https://github.com/stackhpc/kayobe-automation>`_ are now
+    published to GitHub Container Registry (GHCR) at
+    ghcr.io/stackhpc/stackhpc-kayobe-config. The image is tagged with the name
+    of the release branch, e.g. ``stackhpc/yoga``.