Add Ubuntu image for CI
This pulls the 'rolling' tag from https://hub.docker.com/_/ubuntu which seems to be the latest stable release. The 'latest' tag seems to be the latest LTS release, but let's prefer the newest version.
This pulls the 'rolling' tag from https://hub.docker.com/_/ubuntu which seems to be the latest stable release. The 'latest' tag seems to be the latest LTS release, but let's prefer the newest version.