Tried again some minutes ago and I got this
WARNING: We are about to start downloading the Discourse base image
This process may take anywhere between a few minutes to an hour, depending on your network speed
Please be patient
Unable to find image ‘discourse/discourse:1.3.10’ locally
Repository docker.io/discourse/discourse already being pulled by another client. Waiting.
Last time I ran the command, it tried to download the image and got stuck on a Pull Complete message for hours. I guess it is still running.