approx based package proxy running on port 9999
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.