parser: avoid creating temp error string

This error is always ignored by caller and only used
as nil check. Could keep previous output and move Printf
to Error() method but not needed as nothing is actually
reading the original input.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/parser/line_parsers.go

@@ -17,6 +17,7 @@ import (
 
 var (
 	errDockerfileNotStringArray = errors.New("when using JSON array syntax, arrays must be comprised of strings only")
+	errDockerfileNotJSONArray   = errors.New("not a JSON array")
 )
 
 const (
@@ -277,7 +278,7 @@ func parseString(rest string, d *directives) (*Node, map[string]bool, error) {
 func parseJSON(rest string) (*Node, map[string]bool, error) {
 	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
 	if !strings.HasPrefix(rest, "[") {
-		return nil, nil, errors.Errorf("Error parsing %q as a JSON array", rest)
+		return nil, nil, errDockerfileNotJSONArray
 	}
 
 	var myJSON []interface{}