So I actually remember running into the "Unable to locate package docker-engine" issue a while ago, and it seemed like an issue on Docker's end because for me, the issue actually only lasted about half a day before it started working again without any changes on my part.
So I think in the end this was just a case of really unfortunate timing, because if it weren't for the Docker installation issue, the only real complaint left in this post would have been the fact that the official method of installing Docker was to curl a script and pipe into sh[1]. And the rest of the post would have been singing praises of how amazing Docker is to be able to take setting up a Rails app along with all its dependencies and turn it into such a simple, painless process.
[1] Which is a perfectly valid criticism, by the way. They should really document the much saner method of installing through their official repos: