Allow triggering the build workflow manually of needed

There was an issue in github which is probably the root cause why
an image for a merged PR was not running. This PR allows to
trigger the job from the Actions tab in github.

Signed-off-by: Martin Schuppert <mschuppert@redhat.com>

Author: stuggi

.github/workflows/build-openstack-operator.yaml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - '*'
+  workflow_dispatch:  # This allows manual triggering from the Actions tab if needed
 
 env:
   imageregistry: 'quay.io'