If you’re building system images, you’re going to do a lot of debootstrap, which is going to fetch a lot of packages. On a fast system, that’ll be the slowest part of the process. Here’s how to cache.
### Install apt-cacher-ng
sudo apt-get install squid-deb-proxy
### Tell programs to use the proxy
export http_proxy=http://127.0.0.1:8000
# Note that you'll need to re-export this before any use of debootstrap