Export repository scope helper functions

`docker.Authorizer` requires library clients to configure scope via context.
It is helpful for the clients to use the helper (currently private) functions
for generating scope string and to use that function with the combination of
other scope-related ones (e.g. `docker.WithScope`).

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

remotes/docker/fetcher.go

@@ -45,7 +45,7 @@ func (r dockerFetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.R
 		return nil, errors.Wrap(errdefs.ErrNotFound, "no pull hosts")
 	}
 
-	ctx, err := contextWithRepositoryScope(ctx, r.refspec, false)
+	ctx, err := ContextWithRepositoryScope(ctx, r.refspec, false)
 	if err != nil {
 		return nil, err
 	}