For apt to use local cache - apt-cacher-ng
Sorry, I misunderstood.
What you want is a personal repository with no traffic to the internet. Even better, it's already been done, documented, and it's up on the Ubuntu wiki.
Ubuntu Wiki - Personal Repository
Part of the directions for a personal repository is changing the sources.list to a file path. If there are no network URLs (http/https/ftp) then apt will not go on the network. It will only look in the locations listed in the sources.list file.