Bump containerd to v1.6.4 (#151)

Signed-off-by: Luca Comellini <luca.com@gmail.com>

go.mod

@@ -3,12 +3,12 @@ module oras.land/oras-go
 go 1.17
 
 require (
-	github.com/containerd/containerd v1.6.1
+	github.com/containerd/containerd v1.6.4
 	github.com/distribution/distribution/v3 v3.0.0-20211118083504-a29a3c99a684
 	github.com/docker/cli v20.10.11+incompatible
 	github.com/docker/docker v20.10.11+incompatible
 	github.com/opencontainers/go-digest v1.0.0
-	github.com/opencontainers/image-spec v1.0.2
+	github.com/opencontainers/image-spec v1.0.3-0.20211202183452-c5a74bcca799
 	github.com/phayes/freeport v0.0.0-20180830031419-95f893ade6f2
 	github.com/pkg/errors v0.9.1
 	github.com/sirupsen/logrus v1.8.1
@@ -48,7 +48,7 @@ require (
 	github.com/moby/term v0.0.0-20210610120745-9d4ed1856297 // indirect
 	github.com/morikuni/aec v1.0.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/prometheus/client_golang v1.11.0 // indirect
+	github.com/prometheus/client_golang v1.11.1 // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.30.0 // indirect
 	github.com/prometheus/procfs v0.7.3 // indirect