buildContainerImage: Handle null arguments

If the `buildContainerImage` method is called without passing any
arguments at all, the `args` object will be `null`.  Attempting to
access the properties of it in that case raises a
`NullPointerException`.  Fortunately, Groovy has a `?` operator that
returns `null` when the named object is `null` and avoids accessing its
properties.

vars/buildContainerImage.groovy

@@ -5,11 +5,11 @@ def call(args) {
         pipelineTriggers([cron('H H H * *')])
     ])
 
-    def registry = args.registry
-    def project = args.project
-    def name = args.name
-    def tag = args.tag
-    def arch = args.arch
+    def registry = args?.registry
+    def project = args?.project
+    def name = args?.name
+    def tag = args?.tag
+    def arch = args?.arch
     if (registry == null) {
         registry = 'git.pyrocufflink.net'
     }