Make unique snapshotter opt for label-assisted remapping

Provide a snapshotter opt to add labels used by any supporting
snapshotter to handle user namespace filesystem remapping. Currently
supported by the fuse-overlayfs snapshotter, and others can use this
information as well.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

snapshotter_opts_unix.go

@@ -0,0 +1,35 @@
+// +build !windows
+
+/*
+   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 containerd
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/snapshots"
+)
+
+// WithRemapperLabels creates the labels used by any supporting snapshotter
+// to shift the filesystem ownership (user namespace mapping) automatically; currently
+// supported by the fuse-overlayfs snapshotter
+func WithRemapperLabels(ctrUID, hostUID, ctrGID, hostGID, length uint32) snapshots.Opt {
+	return snapshots.WithLabels(map[string]string{
+		"containerd.io/snapshot/uidmapping": fmt.Sprintf("%d:%d:%d", ctrUID, hostUID, length),
+		"containerd.io/snapshot/gidmapping": fmt.Sprintf("%d:%d:%d", ctrGID, hostGID, length),
+	})
+}