follow up on #45 to set a build_ref on deployment hook

previously we populated build_slug to shuffle around via minio

cabotage/celery/tasks/github.py

@@ -103,6 +103,7 @@ def process_deployment_hook(hook):
                 "installation_id": installation_id,
                 "auto_deploy": True,
             },
+            build_ref=deployment["sha"],
         )
         db.session.add(image)
         db.session.flush()