Try setting the proxy. Right click on the docker icon in system tray, go to settings, proxy and add the below settings:

"HTTPS_PROXY=http://<username>:<password>@<host>:<port>"

If you are looking to set a proxy on Linux, see here

The answer of Alexandre Mélard at question Cannot download Docker images behind a proxy works, here is the simplified version:

1. Find out the systemd script or init.d script path of the docker service by running:service docker status or systemctl status docker, for example in Ubuntu16.04 it's at /lib/systemd/system/docker.service

2. Edit the script for example sudo vim /lib/systemd/system/docker.service by adding the following in the [Service] section:

    Environment="HTTP_PROXY=http://<proxy_host>:<port>"
    Environment="HTTPS_PROXY=http://<proxy_host>:<port>"
    Environment="NO_PROXY=<no_proxy_host_or_ip>,<e.g.:172.10.10.10>"
   

3. Reload and restart the daemon: sudo systemctl daemon-reload && sudo systemctl restart docker or sudo service docker restart

4. Verify: docker info | grep -i proxy should show something like:

   HTTP Proxy: http://10.10.10.10:3128
   HTTPS Proxy: http://10.10.10.10:3128
   

5. This adds the proxy for docker pull, which is the problem of the question. If for running or building docker a proxy is needed, either configure ~/.docker/config as the official docs explained, or change Dockerfile so there is proxy inside the container.

I had the same problem on a windows server and solved the problem by setting the environment variable HTTP_PROXY on powershell:

[Environment]::SetEnvironmentVariable("HTTP_PROXY", "http://username:password@proxy:port/", [EnvironmentVariableTarget]::Machine)

And then restarting docker:

Restart-Service docker

More information at Microsoft official proxy-configuration guide.

Note: The error returned when pulling image, with version 19.03.5, was connection refused.