dependency: update docker version to 2.8.0

Impact:
Systems that rely on digest equivalence for image attestations may
be vulnerable to type confusion.

Affected versions:
< 2.8.0

go.mod

@@ -15,7 +15,7 @@ require (
 	github.com/containerd/containerd v1.5.9 // indirect
 	github.com/dave/jennifer v1.4.1
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/docker/distribution v2.7.1+incompatible // indirect
+	github.com/docker/distribution v2.8.0+incompatible // indirect
 	github.com/docker/docker v20.10.7+incompatible
 	github.com/docker/go-connections v0.4.0 // indirect
 	github.com/docker/go-units v0.4.0 // indirect

go.sum

@@ -244,8 +244,9 @@ github.com/dgryski/go-sip13 v0.0.0-20181026042036-e10d5fee7954/go.mod h1:vAd38F8
 github.com/dnaeon/go-vcr v1.0.1/go.mod h1:aBB1+wY4s93YsC3HHjMBMrwTj2R9FHDzUr9KyGc8n1E=
 github.com/docker/distribution v0.0.0-20190905152932-14b96e55d84c/go.mod h1:0+TTO4EOBfRPhZXAeF1Vu+W3hHZ8eLp8PgKVZlcvtFY=
 github.com/docker/distribution v2.7.1-0.20190205005809-0d3efadf0154+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
-github.com/docker/distribution v2.7.1+incompatible h1:a5mlkVzth6W5A4fOsS3D2EO5BUmsJpcB+cRlLU7cSug=
 github.com/docker/distribution v2.7.1+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
+github.com/docker/distribution v2.8.0+incompatible h1:l9EaZDICImO1ngI+uTifW+ZYvvz7fKISBAKpg+MbWbY=
+github.com/docker/distribution v2.8.0+incompatible/go.mod h1:J2gT2udsDAN96Uj4KfcMRqY0/ypR+oyYUYmja8H+y+w=
 github.com/docker/docker v20.10.7+incompatible h1:Z6O9Nhsjv+ayUEeI1IojKbYcsGdgYSNqxe1s2MYzUhQ=
 github.com/docker/docker v20.10.7+incompatible/go.mod h1:eEKB0N0r5NX/I1kEveEz05bcu8tLC/8azJZsviup8Sk=
 github.com/docker/go-connections v0.4.0 h1:El9xVISelRB7BuFusrZozjnkIM5YnzCViNKohAFqRJQ=