packageproxy

The packageproxy snap ships a pre-confgured Approx install. In the default configuration you can point your sources.list to:

 http://localhost:9999/ubuntu

.. or replace "localhost" with the actual IP when accessing it from another host. If you use any of the Ubuntu Ports architectures, replace "/ubuntu" with "ubuntu-ports".

Approx is an HTTP-based proxy server for Debian-style package archives. It fetches files from remote repositories on demand, and caches them for local use.