Allow the image to optionally use a local DNS resolver

[leophys]

To enhance the degree of anonymity of the infrastructure, allow the image to use a local cache.

Explore the possibility to:

• use the same image to host a local dns resolver (thinking of unbound);
• use docker in swarm mode and include a local resolver image.

Resources

unbound images

• https://hub.docker.com/r/mvance/unbound/
• https://hub.docker.com/r/tiredofit/unbound/
• https://github.com/folhabranca/docker-unbound (with DNSSEC validation!)