Update github.com/containerd/containerd to v1.5.7

... to include a fix for
https://github.com/advisories/GHSA-c2h3-6mxw-7mvq .

(Note that Skopeo doesn't depend on the vulnerable code,
so this is primarily to avoid dependency checker warnings.)

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/google.golang.org/protobuf/internal/impl/legacy_message.go

@@ -440,6 +440,13 @@ func legacyMerge(in piface.MergeInput) piface.MergeOutput {
 	if !ok {
 		return piface.MergeOutput{}
 	}
+	if !in.Source.IsValid() {
+		// Legacy Marshal methods may not function on nil messages.
+		// Check for a typed nil source only after we confirm that
+		// legacy Marshal/Unmarshal methods are present, for
+		// consistency.
+		return piface.MergeOutput{Flags: piface.MergeComplete}
+	}
 	b, err := marshaler.Marshal()
 	if err != nil {
 		return piface.MergeOutput{}