Implement new driver-opt: default-load

This eases build driver migrations, as it allows aligning the default behavior.
See also https://docs.docker.com/build/drivers/

Signed-off-by: Niklas Gehlen <niklas@namespacelabs.com>

driver/docker/driver.go

@@ -93,6 +93,7 @@ func (d *Driver) Features(ctx context.Context) map[driver.Feature]bool {
 			driver.DockerExporter: useContainerdSnapshotter,
 			driver.CacheExport:    useContainerdSnapshotter,
 			driver.MultiPlatform:  useContainerdSnapshotter,
+			driver.DefaultLoad:    true,
 		}
 	})
 	return d.features.list