Check out example codes for "docker pull command". It will help you in understanding the concepts better.

Code Example 1

For those trying to create a Google Cloud instance using the "Deploy a container image to this VM instance." option then the correct url format would be

docker.io/<dockerimagename>:version

The suggestion above of registry.hub.docker.com/library/<dockerimagename> did not work for me.

I finally found the solution here (in my case, i was trying to run docker.io/tensorflow/serving:latest)

Code Example 2

docker pull ubuntu:14.04

docker pull ubuntu
docker pull fedora

NOTE:
Docker Hub contains many pre-built images that you can pull
docker pull <Image>:<tag>
docker pull <Image>

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