builder/dockerfile: improve error message about build stage name

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

builder/dockerfile/dispatchers.go

@@ -211,7 +211,7 @@ func from(b *Builder, args []string, attributes map[string]bool, original string
 			return errors.Errorf("invalid name for build stage: %q, name can't start with a number or contain symbols", ctxName)
 		}
 	} else if len(args) != 1 {
-		return errExactlyOneArgument("FROM")
+		return errors.New("FROM requires either one or three arguments")
 	}
 
 	if err := b.flags.Parse(); err != nil {

builder/dockerfile/dispatchers_test.go

@@ -20,7 +20,6 @@ type commandWithFunction struct {
 func TestCommandsExactlyOneArgument(t *testing.T) {
 	commands := []commandWithFunction{
 		{"MAINTAINER", func(args []string) error { return maintainer(nil, args, nil, "") }},
-		{"FROM", func(args []string) error { return from(nil, args, nil, "") }},
 		{"WORKDIR", func(args []string) error { return workdir(nil, args, nil, "") }},
 		{"USER", func(args []string) error { return user(nil, args, nil, "") }},
 		{"STOPSIGNAL", func(args []string) error { return stopSignal(nil, args, nil, "") }}}