I forked the phusion-baseimage just to be able to start testing against Ubuntu 18.04, and will be pushing this to unilynx/phusion-baseimage-1804:1.0.0 on dockerhub
Feel free to use that image if you want to start testing against the LTS beta, but you probably shouldn't use it in production until the guys on https://github.com/phusion/baseimage-docker release a proper image they actually dare vouch for.