For those who have this problem, it is typically related to having an issue with your DNS being unable to resolve index.docker.io. I had this issue today working from home where my internet connection has a default DNS server that is notoriously flakey.
My dev environment is OSX and I easily solved the issue by changing my DNS servers in network settings to Google’s DNS servers (8.8.8.8 and 8.8.4.4) and then restarting my docker host through docker-machine restart MACHINENAME