Fix CoreDNS user

[b-m-f]

Currently coreDNS is running as root inside the container so that it can bind to port 53.

The root user can do this because it can get the capability CAP_NET_BIND_SERVICE.

Maybe there is a way to give this capability to the coredns user inside the container.

Additional steps

If the user has the correct capability the coredns binary must be run as the coredns user by changing

coredns -conf /etc/coredns/Corefile  

to

s6-setuidgid coredns coredns -conf /etc/coredns/Corefile

in /Build-Automation/WirtBot/service-files/dns