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
  update-type: version-update:semver-minor
...

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

vendor/github.com/docker/distribution/registry/client/auth/challenge/authchallenge.go

@@ -117,8 +117,8 @@ func init() {
 		var t octetType
 		isCtl := c <= 31 || c == 127
 		isChar := 0 <= c && c <= 127
-		isSeparator := strings.IndexRune(" \t\"(),/:;<=>?@[]\\{}", rune(c)) >= 0
-		if strings.IndexRune(" \t\r\n", rune(c)) >= 0 {
+		isSeparator := strings.ContainsRune(" \t\"(),/:;<=>?@[]\\{}", rune(c))
+		if strings.ContainsRune(" \t\r\n", rune(c)) {
 			t |= isSpace
 		}
 		if isChar && !isCtl && !isSeparator {