Use DefaultAgentImage for the BuildFailureJob

Previously, the agent image configured in the controller's config was
used for the BuildFailureJob. But customers could have configured this
to be a private image. We don't want to copy the imagePullSecrets in as
there could be an error in their formatting, and the failure job is
designed to bubble up such errors to the job logs. So we use the default
agent image which is guaranteed to be public.

internal/controller/scheduler/scheduler.go

@@ -501,7 +501,17 @@ func (w *jobWrapper) BuildFailureJob(err error) (*batchv1.Job, error) {
 		PodSpec: &corev1.PodSpec{
 			Containers: []corev1.Container{
 				{
-					Image:   w.cfg.Image,
+					// the configured agent image may be private. If there is an error in specifying the
+					// secrets for this image, we should still be able to run the failure job. So, we
+					// bypass the potentially private image and use a public one. We could use a
+					// thinner public image like `alpine:latest`, but it's generally unwise to depend
+					// on an image that's not published by us.
+					//
+					// TODO: pin the version of the agent image and use that here.
+					// Currently, DefaultAgentImage has a latest tag. That's not ideal as
+					// a given version of agent stack-k8s may use different versions of the agent image over
+					// time. We should consider using a specific version of the agent image here.
+					Image:   config.DefaultAgentImage,
 					Command: []string{fmt.Sprintf("echo %q && exit 1", err.Error())},
 				},
 			},