A custom mirror can't work anymore, since we use our own APT snapshots.
AFAICT the documented squid-deb-proxy setup has bitrotted: it hasn't
been maintained for many years, and I strongly doubt it still works.
I won't bother updating it wrt. the "Avoid re-downloading everything one
has in their local caching proxy, and filling its cache with files
duplicated many times" problem.