Updated docker version due to dependabot warning

Author: aregm

go.mod

@@ -1,7 +1,7 @@
 module github.com/intel-go/nff-go
 
 require (
-	github.com/docker/distribution v2.7.1+incompatible // indirect
+	github.com/docker/distribution v2.8.0 // indirect
 	github.com/docker/docker v1.13.1
 	github.com/docker/go-connections v0.4.0
 	github.com/docker/go-units v0.3.3 // indirect