It is currently not supported (or not easy/straightforward) to run (and use with Docker) a DNS server in a container, nor to use an APT cache server container for both builds and container runtime. This feature request is about the general use-case of a custom DNS server to be used during builds.

Let's depict one of the scenarios:

• you have a container that runs your APT-cacher*
• this can provide a good benefit during docker image builds and anyways during packages installation of running containers.
• the IPv4 (Docker) network address of the APT cache proxy container is not fixed, can change anytime during orchestration changes, thus its resolution cannot reasonably be hardcoded in Dockerfile's.

Proposed solution: having an option like --dns or --addn-hosts** would allow to provide dynamically the resolution for such APT-cacher server. This would ease solving this problem and avoid relying on solutions like https://github.com/skynetservices/skydns

* = other package management cache/proxies also apply
** = like for dnsmasq, this option could allow to specify an addendum file /etc/hosts