I can see, however, a benefit in encapsulating different services in different containers, as this potentially gives you some control over them (available disk space, network usage etc.) * . On top of that, I imagine starting out on a "machine agnostic" approach can be rather useful if you have to change your network landscape further down the line: If your database already is configured as if it were running on its own server, there's certainly a bunch of unintended coupling effects you can avoid.
That said, I can't see Docker being the silver bullet it gets hyped up to be sometimes. But that goes for most new and shiny things in the tech space...
* And yes, that's already feasible without containers. Docker's approach to this seems to provide a way to do it in a much more automated way than most alternatives though.
* https://lists.debian.org/debian-user/2017/04/msg00151.html
* http://www.panix.com/~rst/tuttle.html
* https://lists.debian.org/debian-user/2017/04/msg00437.html
Now it's all docker rather than packages, ansible (which leaves no trace of what it's doing on the target machine) rather than a for I in 'cat hosts'. Fine, but where's the benefit?