add example for docker-containers option

[davidak]

Issue description

docker-containers has no example value. please add one so one knows how to use it.

cc @copumpkin

Steps to reproduce

https://nixos.org/nixos/options.html#docker-containers

[kevin-hanselman]

Friendly bump! I'm looking to start some containers on boot, and I feel like this might be the place?

[copumpkin]

This was an experimental feature that I merged by accident. It's been backed out of master but I didn't realize it was in 17.09 too. It shouldn't affect anything for users. See #26075 for more details.

[davidak]

It is not present anymore in 18.03, so this issue is fixed.

@kevlar1818 have a look at #37553