sudo mkdir -p /etc/systemd/system/docker.service.d
sudo vi /etc/systemd/system/docker.service.d/http-proxy.conf
[Service]
Environment="HTTP_PROXY=http://proxy.example.com:3128"
Environment="HTTPS_PROXY=https://proxy.example.com:3129"
Environment="NO_PROXY=localhost,127.0.0.1,docker-registry.example.com,.corp" #if needed
sudo systemctl daemon-reload
sudo systemctl restart docker
sudo systemctl show --property=Environment docker
Output should be something similar to -
Environment=HTTP_PROXY=http://proxy.example.com:3128 HTTPS_PROXY=https://proxy.example.com:3129 NO_PROXY=localhost,127.0.0.1,docker-registry.example.com,.corp
Read more at Docker Documentation website.