Update Image strings before writing them out

If the YAML file contains:
  - path: etc/linuxkit.yml
    metadata: yaml
in the fil section, the image was build with content trust,
then the linuxkit.yml file image contains fully qualified
image references (including the sha256).

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

src/moby/build.go

@@ -388,6 +388,8 @@ func tarAppend(iw *tar.Writer, tr *tar.Reader) error {
 
 // this allows inserting metadata into a file in the image
 func metadata(m Moby, md string) ([]byte, error) {
+	// Make sure the Image strings are update to date with the refs
+	updateImages(&m)
 	switch md {
 	case "json":
 		return json.MarshalIndent(m, "", "    ")

src/moby/config.go

@@ -197,6 +197,31 @@ func extractReferences(m *Moby) error {
 	return nil
 }
 
+func updateImages(m *Moby) error {
+	if m.Kernel.ref != nil {
+		m.Kernel.Image = m.Kernel.ref.String()
+	}
+	for i, ii := range m.initRefs {
+		m.Init[i] = ii.String()
+	}
+	for _, image := range m.Onboot {
+		if image.ref != nil {
+			image.Image = image.ref.String()
+		}
+	}
+	for _, image := range m.Onshutdown {
+		if image.ref != nil {
+			image.Image = image.ref.String()
+		}
+	}
+	for _, image := range m.Services {
+		if image.ref != nil {
+			image.Image = image.ref.String()
+		}
+	}
+	return nil
+}
+
 // NewConfig parses a config file
 func NewConfig(config []byte) (Moby, error) {
 	m := Moby{}