custom fact to know resolver

With Ubuntu1804 systemd-resolved.service manage the file /etc/resolv.conf.

By default, systemd-resolved uses nameserver 127.0.0.53. systemd-resolved provides a local DNS stub listener on IP address 127.0.0.53 on the local loopback interface. So inside the container, the ip address is 127.0.0.53 can not be used and DNS resolution does not work.

The file /etc/resolv.conf is a link to ../run/systemd/resolve/stub-resolv.conf. But systemd-resolved maintains also /usr/lib/systemd/resolv.conf file for compatibility with traditional Linux programs. This file may be symlinked from /etc/resolv.conf and is always kept up-to-date, containing information about all known DNS servers.