Fuzzing: Add 4 fuzzers

Signed-off-by: AdamKorcz <adam@adalogics.com>

contrib/fuzz/docker_fuzzer.go

@@ -29,6 +29,8 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+
+	refDocker "github.com/containerd/containerd/reference/docker"
 )
 
 func FuzzFetcher(data []byte) int {
@@ -76,3 +78,8 @@ func FuzzFetcher(data []byte) int {
 	}
 	return 1
 }
+
+func FuzzParseDockerRef(data []byte) int {
+	_, _ = refDocker.ParseDockerRef(string(data))
+	return 1
+}