Only trigger imagebuilder on relevant repos

Change-Id: Id59296a2d23b397184f13072200b1b01f8018a98
diff --git a/jjb/imagebuilder.yaml b/jjb/imagebuilder.yaml
index 968a702..f9f185d 100644
--- a/jjb/imagebuilder.yaml
+++ b/jjb/imagebuilder.yaml
@@ -7,7 +7,7 @@
     # add repos that have documentation to the project list in both jobs
     jobs:
       - 'publish-imagebuilder':
-          project-regexp: '.*'
+          project-regexp: '{imagebuilder-projects-regexp}'
           branch-regexp: '{supported-branches-regexp}'