I am having the same issue, i have to say i am using Docker for the first time.
In my case, i executed all commands with my non-root sudo user. But i am getting a 503 server error. I am running it on a remote Ubuntu 16 server.
I installed docker on my machine like described here: https://www.digitalocean.com/community/tutorials/how-to-install-and-use-docker-on-ubuntu-16-04