Esta documentação indica que:
As of Docker 1.10 User Namespaces are supported directly by the docker daemon. This feature allows for the root user in a container to be mapped to a non uid-0 user outside the container, which can help to mitigate the risks of container breakout. This facility is available but not enabled by default.
Refer to the daemon command in the command line reference for more information on this feature. Additional information on the implementation of User Namespaces in Docker can be found in this blog post.