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Create file /etc/apt/apt.conf.d/01proxy with content like this:

Acquire::http::Proxy::external.repo.host "http://your.proxy.host/";

With the following settings all http requests to repositories on external.repo.host would be proxied via http proxy at your.proxy.host.

Example for Tailscale:

Acquire::http::Proxy::pkgs.tailscale.com "http://127.0.0.1:8080";