The easy one to fix is "Ubutu Rails" should of course be spelt "Ubuntu Rails".
The other (which will require proper testing) is that the example uses Ubuntu 12.04, which isn't a valid option in 2021. The current latest Ubuntu LTS version is Ubuntu 20.04, but the rest of the example will need testing to ensure that no other changes to the Dockerfile are needed.
The page https://github.com/docker/get-involved/blob/master/content/en/docs/CommunityLeaders/EventHandbooks/Docker101/dockerfile/_index.md#ubutu-rails has a couple of issues.
The easy one to fix is "Ubutu Rails" should of course be spelt "Ubuntu Rails".
The other (which will require proper testing) is that the example uses Ubuntu 12.04, which isn't a valid option in 2021. The current latest Ubuntu LTS version is Ubuntu 20.04, but the rest of the example will need testing to ensure that no other changes to the Dockerfile are needed.