Always add compressed blobs to Docker import manifest

Ensure the manifest which gets created using the Docker
compatibility code compresses the blob before creating
the manifest. This ensures consistency with manifests
used by Docker.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

import_test.go

@@ -17,6 +17,7 @@
 package containerd
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"io"
@@ -26,6 +27,7 @@ import (
 	"runtime"
 	"testing"
 
+	"github.com/containerd/containerd/archive/compression"
 	"github.com/containerd/containerd/archive/tartest"
 	"github.com/containerd/containerd/images"
 	"github.com/containerd/containerd/images/archive"
@@ -274,6 +276,16 @@ func createContent(size int64, seed int64) ([]byte, digest.Digest) {
 	if err != nil {
 		panic(err)
 	}
+	wb := bytes.NewBuffer(nil)
+	cw, err := compression.CompressStream(wb, compression.Gzip)
+	if err != nil {
+		panic(err)
+	}
+
+	if _, err := cw.Write(b); err != nil {
+		panic(err)
+	}
+	b = wb.Bytes()
 	return b, digest.FromBytes(b)
 }