CI: Add Rocky Linux 9 to Kolla container build workflow

Author: markgoddard

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

@@ -23,6 +23,11 @@ on:
         type: boolean
         required: false
         default: true
+      rocky-linux-9:
+        description: Build Rocky Linux 9 images?
+        type: boolean
+        required: false
+        default: true
       ubuntu-focal:
         description: Build Ubuntu Focal 20.04 images?
         type: boolean
@@ -75,6 +80,10 @@ jobs:
             echo -n "$comma\"centos\"" >> $GITHUB_OUTPUT
             comma=", "
           fi
+          if [[ ${{ inputs.rocky-linux-9 }} == 'true' ]]; then
+            echo -n "$comma\"rocky\"" >> $GITHUB_OUTPUT
+            comma=", "
+          fi
           if [[ ${{ inputs.ubuntu-focal }} == 'true' ]]; then
             echo -n "$comma\"ubuntu\"" >> $GITHUB_OUTPUT
             comma=", "