optional apt-get caching for docker

docker-compose.yml

aptcache:
  image: sameersbn/apt-cacher-ng:latest
  ports:
    - "3142:3142"
  volumes:
     - /srv/docker/apt-cacher-ng:/var/cache/apt-cacher-ng
  restart: always

Dockerfile

# configure apt to not install reccomendations
ENV DEBIAN_FRONTEND noninteractive
RUN echo 'APT::Install-Recommends 0;' >> /etc/apt/apt.conf.d/01norecommends \
 && echo 'APT::Install-Suggests 0;' >> /etc/apt/apt.conf.d/01norecommends

RUN apt-get update \
 && apt-get install -y netcat \
 && rm -rf /var/lib/apt/lists/*

# This uses the apt proxy only if present. It checks on port 3142 of the host/gateway, and returns DIRECT for no proxy,
# or the proxy address if it's detected. This uses an undocumented feature of apt-get.
# Ref: http://askubuntu.com/questions/53443/how-do-i-ignore-a-proxy-if-not-available
RUN export HOST_IP=$(ip route| awk '/^default/ {print $3}')\
 && echo "#!/bin/bash" > /usr/local/bin/apt-ng-host-discover \
 && echo "if nc -w1 -z $HOST_IP 3142; then printf http://$HOST_IP:3142; else printf DIRECT; fi" >> /usr/local/bin/apt-ng-host-discover \
 && chmod +x /usr/local/bin/apt-ng-host-discover \
 && echo 'Acquire::http::Proxy-Auto-Detect "/usr/local/bin/apt-ng-host-discover";' > /etc/apt/apt.conf.d/30proxy
 
# test it by running a verbose statement like this
apt-get -o Acquire::http::Proxy-Auto-Detect apt-get -o Acquire::http::Proxy-Auto-Detect download vim