vendor: update buildkit to v0.17.0-rc2

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/util/gitutil/git_commit.go

@@ -0,0 +1,19 @@
+package gitutil
+
+func IsCommitSHA(str string) bool {
+	if len(str) != 40 {
+		return false
+	}
+
+	for _, ch := range str {
+		if ch >= '0' && ch <= '9' {
+			continue
+		}
+		if ch >= 'a' && ch <= 'f' {
+			continue
+		}
+		return false
+	}
+
+	return true
+}