In my case, running docker run hello-world caused error dial tcp: i/o timeout.
The following repaired the issue for me on Mac OS X El Capitan. Went to System Preferences > Network > Advanced > DNS and added DNS server 8.8.8.8. After that, I restarted Docker for Mac and ran docker run hello-world successfully.
Some Windows users have encountered the same issue and managed to get it working in a same way. See: https://docs.docker.com/docker-for-windows/troubleshoot/#networking-issues