Update github.com/Microsoft/hcsshim to v0.8.15

Fixes #4915

Release info: https://github.com/microsoft/hcsshim/releases/tag/v0.8.15

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>

vendor/github.com/Microsoft/hcsshim/internal/cow/cow.go

@@ -80,4 +80,6 @@ type Container interface {
 	// container to be terminated by some error condition (including calling
 	// Close).
 	Wait() error
+	// Modify sends a request to modify container resources
+	Modify(ctx context.Context, config interface{}) error
 }