After installing on a fresh Ubuntu 18.04.1 LXC container, which is the only uncommon aspect of this install, the installer should report DNS resolution is available.
The installer reports that DNS resolution is not available. Calling ‘pihole status’ also reports the DNS resolution is not available.
I have another Pi-hole install running under a container on Ubuntu 16.04 LTS. When I updated it this morning I got the same behaviour as when installing it under 18.04.