really short version.
edit /etc/apt/apt.conf add:
Acquire::http::Proxy "http://some_proxy:some_port/";
also, if running certbot, add to /usr/lib/systemd/system/certbot.service
[Service] Environment="https_proxy=https://server:port" "http_proxy=http://server:port" Type=oneshot
This seems to get overwritten on upgrades.
