(devena-mono) Propagate stderr from kustomize build

The goal here is to propagate warnings (which is all that's assumed to
written to stderr) so they can e.g. be presented during PRs. For this
reason the warnings are written between two delimiters so they can
always be unambiguously parsed.

Tests weren't added since these would depend on the version of
`kustomize` being run and I don't think it's worth the effort trying to
add setup around that just for this small behaviour change. Sample
output (by adding a `patchesStrategicMerge:` to one of our existing
manifests):

    Found kustomization build dir: services/opslevel-k8s-reconciler/k8s-manifests/base
    Running `kustomize build services/opslevel-k8s-reconciler/k8s-manifests/base`
    Built: services/opslevel-k8s-reconciler/k8s-manifests/base
    ---start Warnings---
    Warnings for: services/opslevel-k8s-reconciler/k8s-manifests/base
    # Warning: 'patchesStrategicMerge' is deprecated. Please use 'patches' instead. Run 'kustomize edit fix' to update your Kustomization automatically.
    ---End warnings---

Ticket: DENA-247

Author: matthewhughes-uw

kustomize-build-dirs/main.go

@@ -235,12 +235,20 @@ func buildManifests(kustomizationRoots []string, rootDir string) (map[string]str
 		kustomizationRoot := kustomizationRoots[i]
 		fmt.Printf("Running `kustomize build %s`\n", kustomizationRoot)
 		group.Go(func() error {
-			manifest, err := kustomizeBuild(filepath.Join(rootDir, kustomizationRoot))
+			manifest, stderr, err := kustomizeBuild(filepath.Join(rootDir, kustomizationRoot))
 			if err != nil {
 				return err
 			}
 			mutex.Lock()
 			fmt.Printf("Built: %s\n", kustomizationRoot)
+			if stderr != "" {
+				fmt.Fprintf(
+					os.Stderr,
+					"---start Warnings---\nWarnings for: %s\n%s---End warnings---\n",
+					kustomizationRoot,
+					stderr,
+				)
+			}
 			manifestMap[kustomizationRoot] = manifest
 			defer mutex.Unlock()
 			return nil
@@ -252,7 +260,7 @@ func buildManifests(kustomizationRoots []string, rootDir string) (map[string]str
 	return manifestMap, nil
 }
 
-func kustomizeBuild(path string) (string, error) {
+func kustomizeBuild(path string) (string, string, error) {
 	var stdout strings.Builder
 	var stderr strings.Builder
 	args := []string{"build", path}
@@ -261,15 +269,15 @@ func kustomizeBuild(path string) (string, error) {
 	cmd.Stderr = &stderr
 
 	if err := cmd.Run(); err != nil {
-		return "", fmt.Errorf(
+		return "", "", fmt.Errorf(
 			"Error running 'kustomize %s': %v\nstderr: %s",
 			strings.Join(args, " "),
 			err,
 			stderr.String(),
 		)
 	}
 
-	return stdout.String(), nil
+	return stdout.String(), stderr.String(), nil
 }
 
 func writeManifest(manifest string, outDir string, manifestPath string) error {