Implement support for digests in imageTags

This commit adds a new field to imageTags transformer
so that digests can be used instead of image tags if needed.

Closes https://github.com/kubernetes-sigs/kustomize/issues/326

pkg/transformers/imagetag.go

@@ -18,7 +18,6 @@ package transformers
 
 import (
 	"regexp"
-	"strings"
 
 	"github.com/kubernetes-sigs/kustomize/pkg/resmap"
 	"github.com/kubernetes-sigs/kustomize/pkg/types"
@@ -83,7 +82,12 @@ func (pt *imageTagTransformer) updateContainers(obj map[string]interface{}, path
 		}
 		for _, imagetag := range pt.imageTags {
 			if isImageMatched(image.(string), imagetag.Name) {
-				container["image"] = strings.Join([]string{imagetag.Name, imagetag.NewTag}, ":")
+				container["image"] = imagetag.Name + ":" + imagetag.NewTag
+
+				if imagetag.Digest != "" {
+					container["image"] = imagetag.Name + "@" + imagetag.Digest
+				}
+
 				break
 			}
 		}

pkg/transformers/imagetag_test.go

@@ -50,6 +50,10 @@ func TestImageTagTransformer(t *testing.T) {
 									"name":  "nginx",
 									"image": "nginx:1.7.9",
 								},
+								map[string]interface{}{
+									"name":  "replaced-with-digest",
+									"image": "foobar:1",
+								},
 							},
 						},
 					},
@@ -114,6 +118,10 @@ func TestImageTagTransformer(t *testing.T) {
 									"name":  "nginx",
 									"image": "nginx:v2",
 								},
+								map[string]interface{}{
+									"name":  "replaced-with-digest",
+									"image": "foobar@sha256:24a0c4b4a4c0eb97a1aabb8e29f18e917d05abfe1b7a7c07857230879ce7d3d3",
+								},
 							},
 						},
 					},
@@ -160,6 +168,7 @@ func TestImageTagTransformer(t *testing.T) {
 		{Name: "nginx", NewTag: "v2"},
 		{Name: "my-nginx", NewTag: "previous"},
 		{Name: "myprivaterepohostname:1234/my/image", NewTag: "v1.0.1"},
+		{Name: "foobar", Digest: "sha256:24a0c4b4a4c0eb97a1aabb8e29f18e917d05abfe1b7a7c07857230879ce7d3d3"},
 	})
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)