client: Client.ImageSave: close reader on context cancellation

Use a cancelReadCloser to automatically close the reader when the context
is cancelled. Consumers are still recommended to manually close the reader,
but the cancelReadCloser makes the Close idempotent.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

internal/testutil/daemon/daemon.go

@@ -879,7 +879,7 @@ func (d *Daemon) LoadImage(ctx context.Context, t testing.TB, img string) {
 
 	reader, err := clientHost.ImageSave(ctx, []string{img})
 	assert.NilError(t, err, "[%s] failed to download %s", d.id, img)
-	defer reader.Close()
+	defer func() { _ = reader.Close() }()
 
 	c := d.NewClientT(t)
 	defer c.Close()