We're `watchtower:latest-dev` image for the `--schedule` support. This
was only available in the `latest-dev` version of Watchtower.
Now, this feature is available in the `latest` version of Watchtower, so
we should be fine switching back to `latest`.
More importantly, the `latest-dev` version of Watchtower, doesn't have
ARM images built. It only has AMD images built today. So this watchtower
service fails to start on ARM architectures. This PR fixes that.
Added additional details about how to use the fat container, and how to manage / maintain it.
Also includes some refactorings that shouldn't affect the functionality significantly.
