feat: attempt to use dependabot to manage ubuntu updates

[madwort]

• WIP: templating is not implemented, as I want to confirm that we can use dependabot will pick up ubuntu versions correctly before implementing
• intentionally use the tags before the current tag, in order to see whether dependabot opens PRs
• separate tests have raised issues build(deps): bump ubuntu from 20.04 to 22.04 madwort/docker-dependabot-test-2#1 - in this case 20.04 is being updated to 22.04, which could be worked around, but also the new hash is from https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2204-Readme.md , I can't find it in https://hub.docker.com/_/ubuntu/tags
• also, dependabot config only appears to accept directory, not filename, so the separate Dockerfiles will need to be e.g. ubuntu-20.04/Dockerfile not ubuntu-20.04.Dockerfile