Add docker-credential-helpers dependency.

vendor/github.com/docker/docker-credential-helpers/credentials/helper.go

@@ -0,0 +1,14 @@
+package credentials
+
+// Helper is the interface a credentials store helper must implement.
+type Helper interface {
+	// Add appends credentials to the store.
+	Add(*Credentials) error
+	// Delete removes credentials from the store.
+	Delete(serverURL string) error
+	// Get retrieves credentials from the store.
+	// It returns username and secret as strings.
+	Get(serverURL string) (string, string, error)
+	// List returns the stored serverURLs and their associated usernames.
+	List() (map[string]string, error)
+}