upgrade dependency (#34)

Signed-off-by: Shiwei Zhang <shizh@microsoft.com>

go.mod

@@ -2,23 +2,19 @@ module oras.land/oras-go
 
 go 1.16
 
-// WARNING! Do NOT replace these without also replacing their lines in the `require` stanza below.
-// These `replace` stanzas are IGNORED when this is imported as a library
-replace github.com/docker/docker => github.com/moby/moby v20.10.8+incompatible
-
 require (
-	github.com/containerd/containerd v1.5.5
-	github.com/distribution/distribution/v3 v3.0.0-20210826081326-677772e08d64
-	github.com/docker/cli v20.10.8+incompatible
-	github.com/docker/docker v20.10.8+incompatible
+	github.com/containerd/containerd v1.5.7
+	github.com/distribution/distribution/v3 v3.0.0-20210926092439-1563384b69df
+	github.com/docker/cli v20.10.9+incompatible
+	github.com/docker/docker v20.10.9+incompatible
 	github.com/opencontainers/go-digest v1.0.0
 	github.com/opencontainers/image-spec v1.0.1
 	github.com/phayes/freeport v0.0.0-20180830031419-95f893ade6f2
 	github.com/pkg/errors v0.9.1
 	github.com/sirupsen/logrus v1.8.1
-	github.com/spf13/cobra v1.0.0 // indirect
+	github.com/spf13/cobra v1.2.1
 	github.com/stretchr/testify v1.7.0
-	golang.org/x/crypto v0.0.0-20210817164053-32db794688a5
+	golang.org/x/crypto v0.0.0-20210921155107-089bfa567519
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 )