Refactors getRootIDs

Signed-off-by: John Howard <jhoward@microsoft.com>

runtime/container_linux.go

@@ -0,0 +1,22 @@
+package runtime
+
+import "github.com/opencontainers/specs"
+
+func getRootIDs(s *platformSpec) (int, int, error) {
+	if s == nil {
+		return 0, 0, nil
+	}
+	var hasUserns bool
+	for _, ns := range s.Linux.Namespaces {
+		if ns.Type == specs.UserNamespace {
+			hasUserns = true
+			break
+		}
+	}
+	if !hasUserns {
+		return 0, 0, nil
+	}
+	uid := hostIDFromMap(0, s.Linux.UIDMappings)
+	gid := hostIDFromMap(0, s.Linux.GIDMappings)
+	return uid, gid, nil
+}