Ubuntu – How to set up a local Ubuntu Xenial repository using apt-cacher-ng for later network installation

To force the installer to use your proxy server, configure it using preseeding properly using the d-i mirror/http/proxy option, e.g.:

d-i mirror/http/proxy string http://ip-or-hostname-of-proxy:8000/

You don't really need the squid-deb-proxy-client package to use the Squid proxy. The only purpose of the -client package is that it can auto-discover the proxy servers in the network.

Have you tried putting the acng service into offline mode?

After you've updated the dev server, add offlinemode:1 to the acng config file and restart the server. It can still serve the cached content but won't retrieve any newer packages over the Internet.