Add image push to dist tool

Implements image push with progress tracking.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

images/handlers.go

@@ -13,6 +13,8 @@ import (
 
 var SkipDesc = fmt.Errorf("skip descriptor")
 
+var StopHandler = fmt.Errorf("stop handler")
+
 type Handler interface {
 	Handle(ctx context.Context, desc ocispec.Descriptor) (subdescs []ocispec.Descriptor, err error)
 }
@@ -24,12 +26,17 @@ func (fn HandlerFunc) Handle(ctx context.Context, desc ocispec.Descriptor) (subd
 }
 
 // Handlers returns a handler that will run the handlers in sequence.
+//
+// A handler may return `StopHandler` to stop calling additional handlers
 func Handlers(handlers ...Handler) HandlerFunc {
 	return func(ctx context.Context, desc ocispec.Descriptor) (subdescs []ocispec.Descriptor, err error) {
 		var children []ocispec.Descriptor
 		for _, handler := range handlers {
 			ch, err := handler.Handle(ctx, desc)
 			if err != nil {
+				if errors.Cause(err) == StopHandler {
+					break
+				}
 				return nil, err
 			}