pkg/dockerscript: '.' is not a special character

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

pkg/dockerscript/scanner/extra.go

@@ -14,7 +14,7 @@ func detectIdent(ch rune) bool {
 	if unicode.IsDigit(ch) {
 		return true
 	}
-	if strings.ContainsRune("_:/+-@%^", ch) {
+	if strings.ContainsRune("_:/+-@%^.", ch) {
 		return true
 	}
 	return false