Perhaps you need to set up lowercase variables. In my case, my /etc/systemd/system/docker.service.d/http-proxy.conf file looks like this:
[Service]
Environment="ftp_proxy=http://<user>:<password>@<proxy_ip>:<proxy_port>/"
Environment="http_proxy=http://<user>:<password>@<proxy_ip>:<proxy_port>/"
Environment="https_proxy=http://<user>:<password>@<proxy_ip>:<proxy_port>/"
Good luck! :)