bci2: Add buildArgs argument

The `buildArgs` argument to `buildContainerImage2` accepts a list of
arguments that will be passed as `--build-arg` values to the `buildah
build` command.  For example,

```groovy
buildContainerImage2(buildArgs: ['FOO=bar'])
```

Would result in a command like

```sh
buildah build --build-arg FOO=bar ...
```

vars/buildContainerImage2.groovy

@@ -10,6 +10,7 @@ def call(args) {
     def defaultBranch = args?.defaultBranch
     def pi = args?.pi
     def resources = args?.resources
+    def buildArgs = args?.buildArgs
 
     properties([
         pipelineTriggers([cron(schedule ?: 'H H H * *')])
@@ -47,6 +48,7 @@ def call(args) {
                     registry: registry,
                     arch: arch,
                     resources: resources,
+                    buildArgs: buildArgs,
                 )
             }
         }
@@ -95,13 +97,18 @@ def buildStage(args) {
     def registry = args.registry
     def arch = args.arch
     def resources = args?.resources
+    def buildArgs = args?.buildArgs ?: []
+
+    def build_command = "buildah build -t '${full_name}'"
+    buildArgs.each { build_command += "--build-arg '${it}'" }
+    build_command += ' .'
 
     runInPod(arch: arch, resources: resources) {
         checkout scm
 
         container('buildah') {
             withBuildahCreds(registry) {
-                sh "buildah build -t '${full_name}' ."
+                sh build_command
                 sh "buildah push '${full_name}' oci-archive:\${PWD}/${escapeImageName(name)}-${arch}.tar:${full_name}"
                 stash name: arch, includes: "${escapeImageName(name)}-*.tar"
             }