v2 runtime: reduce permissions for bundle dir

Bundle directory permissions should be 0700 by default.  On Linux with
user namespaces enabled, the remapped root also needs access to the
bundle directory.  In this case, the bundle directory is modified to
0710 and group ownership is changed to the remapped root group.

Signed-off-by: Samuel Karp <skarp@amazon.com>

runtime/v2/bundle_test.go

@@ -0,0 +1,23 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package v2
+
+import (
+	// When testutil is imported for one platform (bundle_linux_test.go) it
+	// should be imported for all platforms.
+	_ "github.com/containerd/containerd/pkg/testutil"
+)