avoid double pushing with docker driver with containerd

In this mode buildkit can push directly so pushing manually
with docker would result in pushing image twice.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

driver/docker-container/driver.go

@@ -420,6 +420,7 @@ func (d *Driver) Features(ctx context.Context) map[driver.Feature]bool {
 		driver.DockerExporter: true,
 		driver.CacheExport:    true,
 		driver.MultiPlatform:  true,
+		driver.DirectPush:     true,
 		driver.DefaultLoad:    d.defaultLoad,
 	}
 }