fetch: Use data from descriptor when available.

OCI added support for a `data` field in a descriptor.
This field is expected to contain the content being pointed to by the
descriptor.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

remotes/handlers.go

@@ -17,6 +17,7 @@
 package remotes
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -139,6 +140,10 @@ func Fetch(ctx context.Context, ingester content.Ingester, fetcher Fetcher, desc
 		return err
 	}
 
+	if desc.Size == int64(len(desc.Data)) {
+		return content.Copy(ctx, cw, bytes.NewReader(desc.Data), desc.Size, desc.Digest)
+	}
+
 	rc, err := fetcher.Fetch(ctx, desc)
 	if err != nil {
 		return err