builder/copy-unix: fix filepath.Walk args
Mark unused WalkFunc arguments as such, to get rid of the
following linterwarning:
> builder/dockerfile/copy_unix.go:28:71: SA4009: argument err is overwritten before first use (staticcheck)
> 	return filepath.Walk(source, func(fullpath string, info os.FileInfo, err error) error {
@@ -25,7 +25,7 @@ func fixPermissions(source, destination string, identity idtools.Identity, overr
// We Walk on the source rather than on the destination because we don't
// want to change permissions on things we haven't created or modified.
- return filepath.Walk(source, func(fullpath string, info os.FileInfo, err error) error {
+ return filepath.Walk(source, func(fullpath string, _ os.FileInfo, _ error) error {
// Do not alter the walk root iff. it existed before, as it doesn't fall under
// the domain of "things we should chown".
if skipChownRoot && source == fullpath {
