Bump github.com/docker/distribution

Bumps [github.com/docker/distribution](https://github.com/docker/distribution) from 2.7.1+incompatible to 2.8.0+incompatible.
- [Release notes](https://github.com/docker/distribution/releases)
- [Commits](https://github.com/docker/distribution/compare/v2.7.1...v2.8.0)

---
updated-dependencies:
- dependency-name: github.com/docker/distribution
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/github.com/docker/distribution/reference/normalize.go

@@ -56,6 +56,35 @@ func ParseNormalizedNamed(s string) (Named, error) {
 	return named, nil
 }
 
+// ParseDockerRef normalizes the image reference following the docker convention. This is added
+// mainly for backward compatibility.
+// The reference returned can only be either tagged or digested. For reference contains both tag
+// and digest, the function returns digested reference, e.g. docker.io/library/busybox:latest@
+// sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa will be returned as
+// docker.io/library/busybox@sha256:7cc4b5aefd1d0cadf8d97d4350462ba51c694ebca145b08d7d41b41acc8db5aa.
+func ParseDockerRef(ref string) (Named, error) {
+	named, err := ParseNormalizedNamed(ref)
+	if err != nil {
+		return nil, err
+	}
+	if _, ok := named.(NamedTagged); ok {
+		if canonical, ok := named.(Canonical); ok {
+			// The reference is both tagged and digested, only
+			// return digested.
+			newNamed, err := WithName(canonical.Name())
+			if err != nil {
+				return nil, err
+			}
+			newCanonical, err := WithDigest(newNamed, canonical.Digest())
+			if err != nil {
+				return nil, err
+			}
+			return newCanonical, nil
+		}
+	}
+	return TagNameOnly(named), nil
+}
+
 // splitDockerDomain splits a repository name to domain and remotename string.
 // If no valid domain is found, the default domain is used. Repository name
 // needs to be already validated before.

vendor/github.com/docker/distribution/reference/reference.go

@@ -205,7 +205,7 @@ func Parse(s string) (Reference, error) {
 	var repo repository
 
 	nameMatch := anchoredNameRegexp.FindStringSubmatch(matches[1])
-	if nameMatch != nil && len(nameMatch) == 3 {
+	if len(nameMatch) == 3 {
 		repo.domain = nameMatch[1]
 		repo.path = nameMatch[2]
 	} else {