Support selecting build architecture

The `buildContainerImage` method now supports an optional `arch` keyword
argument.  This argument can be used to select the architecture of the
node running the pod building the container image.  If unspecified, it
defaults to `amd64`.

vars/buildContainerImage.groovy

@@ -9,6 +9,7 @@ def call(args) {
     def project = args.project
     def name = args.name
     def tag = args.tag
+    def arch = args.arch
     if (registry == null) {
         registry = 'git.pyrocufflink.net'
     }
@@ -28,12 +29,18 @@ def call(args) {
             replaceAll('[^a-zA-z0-9._-]', '-').
             replaceAll('^[.-]', '_')
     }
+    if (arch == null) {
+        arch = 'amd64'
+    }
     def repo = "${registry}/${project}/${name}"
     def full_name = "${repo}:${tag}"
 
     def podTemplateYaml = libraryResource('podTemplate.yaml')
 
-    podTemplate(yaml: podTemplateYaml) {
+    podTemplate(
+        yaml: podTemplateYaml,
+        nodeSelector: "kubernetes.io/arch=${arch}",
+    ) {
         node(POD_LABEL) {
             checkout scm