Make email field omitempty for compatibility

Newer instances of auth which do not include an email should not serialize the key,
but existing values should not be stripped on serialization.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

Author: dmcgowan

types/auth.go

@@ -2,9 +2,15 @@ package types
 
 // AuthConfig contains authorization information for connecting to a Registry
 type AuthConfig struct {
-	Username      string `json:"username,omitempty"`
-	Password      string `json:"password,omitempty"`
-	Auth          string `json:"auth,omitempty"`
+	Username string `json:"username,omitempty"`
+	Password string `json:"password,omitempty"`
+	Auth     string `json:"auth,omitempty"`
+
+	// Email is an optional value associated with the username.
+	// This field is deprecated and will be removed in a later
+	// version of docker.
+	Email string `json:"email,omitempty"`
+
 	ServerAddress string `json:"serveraddress,omitempty"`
 	RegistryToken string `json:"registrytoken,omitempty"`
 }