vagrant/website/docs/source/v2/docker/boxes.html.md

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Boxes - Docker Provider	docker-boxes

Docker Boxes

The Docker provider doesn't require a Vagrant box. The config.vm.box setting is completely optional.

A box can still be used and specified, however, to provide defaults. Because the Vagrantfile within a box is loaded as part of the configuration loading sequence, it can be used to configure the foundation of a development environment.

In general, however, you won't need a box with the Docker provider.