vendor: add opencontainers/go-digest to vendor.conf

Signed-off-by: Stephen J Day <stephen.day@docker.com>

vendor/github.com/opencontainers/go-digest/digester.go

@@ -0,0 +1,25 @@
+package digest
+
+import "hash"
+
+// Digester calculates the digest of written data. Writes should go directly
+// to the return value of Hash, while calling Digest will return the current
+// value of the digest.
+type Digester interface {
+	Hash() hash.Hash // provides direct access to underlying hash instance.
+	Digest() Digest
+}
+
+// digester provides a simple digester definition that embeds a hasher.
+type digester struct {
+	alg  Algorithm
+	hash hash.Hash
+}
+
+func (d *digester) Hash() hash.Hash {
+	return d.hash
+}
+
+func (d *digester) Digest() Digest {
+	return NewDigest(d.alg, d.hash)
+}