Remove Unbound dependency to allow arbitrary DNS server

[taoeffect]

I've forked and modified this repo to allow usage with an arbitrary DNS server that runs in a separate container.

I've made it trivial to switch between PowerDNS and Unbound by changing a single line.

If it helps you close this issue, I can grant you access to my private repo where I have these changes made. Just let me know.

My Dockerfile contains only these dependencies:

ENV BUILD_DEPS   build-base autoconf libevent-dev
ENV RUNTIME_DEPS libevent findutils

That's in addition to ca-certificates wget and libressl that come with my custom alpine base image.