fixes #298

cli/cmd/docker.go

@@ -131,6 +131,10 @@ func dockerConfig(user *api.AuthCheckResponse, client *api.Client, args []string
 		return err
 	}
 
+	if config.CredentialHelpers == nil {
+		config.CredentialHelpers = make(map[string]string)
+	}
+
 	for _, regURL := range regToAdd {
 		config.CredentialHelpers[regURL] = "porter"
 	}