The following servers are hardcoded in the generic/ubuntu1804 image:

# systemd-resolve --status
Global
         DNS Servers: 4.2.2.1
                      4.2.2.2
                      208.67.220.220

Wouldn't it be expected that the local DNS from DHCP should have priority over these servers? If running in an environment where access is blocked to external DNS servers, then this image requires further work to use, rather than working out of the box, like say, bento/ubuntu-18.04 or ubuntu/bionic64.