To configure docker (in RHEL) to get images through proxy, edit /etc/sysconfig/docker and add the following parameters according to your environment:
HTTP_PROXY="http://<proxy_host>:<proxy_port>"
HTTPS_PROXY="https://<proxy_host>:<proxy_port>"
http_proxy="${HTTP_PROXY}"
https_proxy="${HTTPS_PROXY}"
And restart docker service:
systemctl restart docker