If you are struggling pulling an image with the
coreos/etd as in the following command:
$ docker pull coreos/etcd
The images is not found, because it is no longer on the official Docker hub. You need point to separate Docker repository hosted on Quay servers. You need to use the following command, instead.
then you need to use
$ docker pull quay.io/coreos/etcd
PS: The repo moved from the Docker to a new repository some time ago.