update error var and add comment

Author: saehejkang

pkg/build/buildopts.go

@@ -246,13 +246,14 @@ func NewBuildOpts(ctx context.Context, basePath string, contextMap map[string][]
 		if !ok {
 			return nil, nil
 		}
+		// Only --ssh default is supported for now, so all other cases are rejected.
 		if len(values) != 1 {
-			return nil, ErrInvalidSSH
+			return nil, ErrUnsupportedSSH
 		}
 
 		value := strings.TrimSpace(values[0])
 		if value != "default" {
-			return nil, ErrInvalidSSH
+			return nil, ErrUnsupportedSSH
 		}
 
 		agentConfigs := make([]sshprovider.AgentConfig, 0, len(values))

pkg/build/errors.go

@@ -28,5 +28,5 @@ var (
 	ErrNoBuildDirectives         = fmt.Errorf("no build directives")
 	ErrInvalidImageContextFormat = fmt.Errorf("image resolver: image name format is invalid")
 	ErrInvalidProgress           = fmt.Errorf("build arg progress value is invalid")
-	ErrInvalidSSH                = fmt.Errorf("build arg ssh value is invalid")
+	ErrUnsupportedSSH            = fmt.Errorf("build arg ssh value is not supported")
 )