Replaced shlex with dockerfile/shell package

pkg/dockerfile/dockerfile.go

@@ -23,12 +23,15 @@ import (
 	"github.com/docker/docker/builder/dockerfile/parser"
 )
 
+var EscapeToken rune
+
 // Parse parses the contents of a Dockerfile and returns a list of commands
 func Parse(b []byte) ([]instructions.Stage, error) {
 	p, err := parser.Parse(bytes.NewReader(b))
 	if err != nil {
 		return nil, err
 	}
+	EscapeToken = p.EscapeToken
 	stages, _, err := instructions.Parse(p.AST)
 	if err != nil {
 		return nil, err