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 68afb33..e60cd48 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -156,3 +156,5 @@
     release:
       jobs:
         - atmosphere-publish-collection
+    vars:
+      container_registry: harbor.atmosphere.dev/library