TLDR: I now add the following snippet to all my Dockerfiles:
# If host is running squid-deb-proxy on port 8000, populate /etc/apt/apt.conf.d/30proxy
# By default, squid-deb-proxy 403s unknown sources, so apt shouldn't proxy ppa.launchpad.net
RUN route -n | awk '/^0.0.0.0/ {print $2}' > /tmp/host_ip.txt
RUN echo "HEAD /" | nc `cat /tmp/host_ip.txt` 8000 | grep squid-deb-proxy \
&& (echo "Acquire::http::Proxy \"http://$(cat /tmp/host_ip.txt):8000\";" > /etc/apt/apt.conf.d/30proxy) \
&& (echo "Acquire::http::Proxy::ppa.launchpad.net DIRECT;" >> /etc/apt/apt.conf.d/30proxy) \
|| echo "No squid-deb-proxy detected on docker host"