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

@@ -100,7 +100,7 @@ def buildStage(args) {
     def buildArgs = args?.buildArgs ?: []
 
     def build_command = "buildah build -t '${full_name}'"
-    buildArgs.each { build_command += " --build-arg '${it}'" }
+    buildArgs.each { build_command += "--build-arg '${it}'" }
     build_command += ' .'
 
     runInPod(arch: arch, resources: resources) {