Introduce container_registry for image prefix

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Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/zuul.d/project.yaml b/zuul.d/project.yaml
index 8675841..d8413e9 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -158,3 +158,5 @@
             dependencies: *molecule_check_dependencies
         - atmosphere-molecule-aio-upgrade-2023.2-ovn:
             dependencies: *molecule_check_dependencies
+    vars:
+      container_registry: harbor.atmosphere.dev/library