unshare: fix creating a userns when running as root

this check would prevent the function to ever be used when running
as root, since we won't check what capabilities are currently
available to the process.

Signed-off-by: Giuseppe Scrivano <[email protected]>
(cherry picked from commit 1af3928)

pkg/unshare/unshare_linux.go

@@ -448,7 +448,7 @@ type Runnable interface {
 // MaybeReexecUsingUserNamespace re-exec the process in a new namespace
 func MaybeReexecUsingUserNamespace(evenForRoot bool) {
 	// If we've already been through this once, no need to try again.
-	if os.Geteuid() == 0 && IsRootless() {
+	if os.Geteuid() == 0 && GetRootlessUID() > 0 {
 		return
 	}