Add cleanup step to remove untagged images by default

vars/buildPodman.groovy

@@ -68,6 +68,13 @@ def call(Map config=[:]) {
           }
         }
 
+        // Basic registry retention removing untagged images
+        stage('cleanup') {
+          when { not { changeRequest() } }
+          steps {
+            sh 'make rm-remote-untagged'
+          }
+        }
       }
     }
   }