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/golang.org/x/net/http2/pipe.go

@@ -30,6 +30,17 @@ type pipeBuffer interface {
 	io.Reader
 }
 
+// setBuffer initializes the pipe buffer.
+// It has no effect if the pipe is already closed.
+func (p *pipe) setBuffer(b pipeBuffer) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.err != nil || p.breakErr != nil {
+		return
+	}
+	p.b = b
+}
+
 func (p *pipe) Len() int {
 	p.mu.Lock()
 	defer p.mu.Unlock()