Releases container layer on export

When running docker export command the container layer
is only released in case there is an error.

This makes the daemon not being able to remove
them when using docker rmi or docker system prune
leaving the container layer and the image used
in an orphaned state on the docker file system.

After applying this patch, the layer is always
released allowing dockerd to remove/prune it.

Signed-off-by: Joan Grau <[email protected]>

daemon/images/image_exporter.go

@@ -25,12 +25,11 @@ func (i *ImageService) PerformWithBaseFS(ctx context.Context, c *container.Conta
 	if err != nil {
 		return err
 	}
+
 	defer func() {
+		err := i.ReleaseLayer(rwlayer)
 		if err != nil {
-			err2 := i.ReleaseLayer(rwlayer)
-			if err2 != nil {
-				log.G(ctx).WithError(err2).WithField("container", c.ID).Warn("Failed to release layer")
-			}
+			log.G(ctx).WithError(err).WithField("container", c.ID).Warn("Failed to release layer")
 		}
 	}()
 
@@ -39,6 +38,8 @@ func (i *ImageService) PerformWithBaseFS(ctx context.Context, c *container.Conta
 		return err
 	}
 
+	defer rwlayer.Unmount()
+
 	return fn(basefs)
 }