Configuration management software is good to have for managing servers because 1) they are mutable, 2) you're not always building from scratch, and 3) you may have to support many different environments (different distros, distro versions, exact same distro with different packages installed...).

Containers get rid of all these problems: Your images are immutable, you can always build from scratch, and you have full control of what goes in the image. Because of this, a plain shell script or something like Dockerfile is much simpler and easier to reason about. I really don't see the point in Ansible Container.