Brings up a parallel CoreDNS instance on ports 11053/19153 with a
single test.example.com zone. Useful for verifying the custom image
builds and the rfc2136 plugin accepts/applies UPDATEs end-to-end
before touching production zones.
Already validated the msgAcceptFunc override fix end-to-end via
nsupdate, with the auto plugin re-serving the new record within 5s.
Note: zones/test.example.com.zone gets rewritten by the plugin
during testing. If perms get hosed (docker writes as root), run
sudo chown -R rpm:rpm test/zones/ to reclaim.
