add reusable container build as trigger file path

Add reusable container build file as trigger file path for the
build-image-action.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>

.github/workflows/build-images-action.yml

@@ -7,6 +7,7 @@ on:
     paths:
     - 'prow/container-images/**/Dockerfile'
     - '.github/workflows/build-images-action.yml'
+    - '.github/workflows/container-image-build.yml'
 
 permissions:
   contents: read