Use user-specific temp directory if set

This allows non-privileged users to use containerd. This is part of a
larger track of work integrating containerd into Cloudfoundry's garden
with support for rootless.

[#156343575]

Signed-off-by: Claudia Beresford <cberesford@pivotal.io>

archive/tar.go

@@ -199,7 +199,7 @@ func applyNaive(ctx context.Context, root string, tr *tar.Reader, options ApplyO
 				basename := filepath.Base(hdr.Name)
 				aufsHardlinks[basename] = hdr
 				if aufsTempdir == "" {
-					if aufsTempdir, err = ioutil.TempDir("", "dockerplnk"); err != nil {
+					if aufsTempdir, err = ioutil.TempDir(os.Getenv("XDG_RUNTIME_DIR"), "dockerplnk"); err != nil {
 						return 0, err
 					}
 					defer os.RemoveAll(aufsTempdir)