archive: add WithSourceDateEpoch() for whiteouts

This makes diff archives to be reproducible.

The value is expected to be passed from CLI applications via the $SOUCE_DATE_EPOCH env var.

See https://reproducible-builds.org/docs/source-date-epoch/
for the $SOURCE_DATE_EPOCH specification.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/snapshots/snapshots.go

@@ -31,6 +31,7 @@ import (
 	"github.com/containerd/containerd/diff"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/pkg/epoch"
 	"github.com/containerd/containerd/pkg/progress"
 	"github.com/containerd/containerd/rootfs"
 	"github.com/containerd/containerd/snapshots"
@@ -142,6 +143,14 @@ var diffCommand = cli.Command{
 			diff.WithLabels(labels),
 		}
 
+		ep, err := epoch.SourceDateEpoch()
+		if err != nil {
+			return err
+		}
+		if ep != nil {
+			opts = append(opts, diff.WithSourceDateEpoch(ep))
+		}
+
 		if idB == "" {
 			desc, err = rootfs.CreateDiff(ctx, idA, snapshotter, client.DiffService(), opts...)
 			if err != nil {