Image relocation

Fixes cnabio#668

Gopkg.toml

@@ -53,3 +53,7 @@
 [[constraint]]
   name = "github.com/docker/go"
   version = "1.5.1-1"
+
+[[constraint]]
+  branch = "master"
+  name = "github.com/pivotal/image-relocation"

cmd/duffle/build.go

@@ -149,15 +149,9 @@ func (b *buildCmd) run() (err error) {
 }
 
 func (b *buildCmd) writeBundle(bf *bundle.Bundle) (string, error) {
-	data, err := json.MarshalCanonical(bf)
+	data, digest, err := marshalBundle(bf)
 	if err != nil {
-		return "", err
-	}
-	data = append(data, '\n') //TODO: why?
-
-	digest, err := digest.OfBuffer(data)
-	if err != nil {
-		return "", fmt.Errorf("cannot compute digest from bundle: %v", err)
+		return "", fmt.Errorf("cannot marshal bundle: %v", err)
 	}
 
 	if b.outputFile != "" {
@@ -169,6 +163,21 @@ func (b *buildCmd) writeBundle(bf *bundle.Bundle) (string, error) {
 	return digest, ioutil.WriteFile(filepath.Join(b.home.Bundles(), digest), data, 0644)
 }
 
+func marshalBundle(bf *bundle.Bundle) ([]byte, string, error) {
+	data, err := json.MarshalCanonical(bf)
+	if err != nil {
+		return nil, "", err
+	}
+	data = append(data, '\n') //TODO: why?
+
+	digest, err := digest.OfBuffer(data)
+	if err != nil {
+		return nil, "", fmt.Errorf("cannot compute digest from bundle: %v", err)
+	}
+
+	return data, digest, nil
+}
+
 func defaultDockerTLS() bool {
 	return os.Getenv(dockerTLSEnvVar) != ""
 }