Added cron trigger for build

Jenkinsfile

@@ -3,6 +3,9 @@ String[] ImgVariantList = ['Default-minmal', 'Default-default']
 
 pipeline {
     agent { label 'LXCBuilder' }
+    triggers {
+        cron '@monthly'
+    }
     parameters {
         booleanParam defaultValue: true, description: 'Builds Default images', name: 'BuildDefault'
         booleanParam defaultValue: false, description: 'Builds Extra images', name: 'BuildExtra'