add triggers

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>

Jenkinsfile

 release = "17.12"
 
+properties(
+    [
+        pipelineTriggers([cron('H H(1-4) * * *')]),
+    ]
+)
+
 def buildImage(architecture, type, board, debosarguments = "", sysroot = false) {
   return {
     node("docker-slave") {