bci2: Allow setting custom schedule

The `buildContainerImage2` function now accepts an optional `schedule`
argument.  If specified, the value will be used as the cron expression
for triggering a build periodically.

vars/buildContainerImage2.groovy

@@ -1,15 +1,16 @@
 // vim: set sw=4 sts=4 ts=4 et :
 
 def call(args) {
-    properties([
-        pipelineTriggers([cron('H H H * *')])
-    ])
-
     def registry = args?.registry
     def project = args?.project
     def name = args?.name
     def tag = args?.tag
     def archlist = args?.archlist
+    def schedule = args?.schedule
+
+    properties([
+        pipelineTriggers([cron(schedule ?: 'H H H * *')])
+    ])
 
     if (registry == null) {
         registry = 'git.pyrocufflink.net'