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We do this mostly because Docker is the only provider that using it requires some amount of Docker-specific knowledge. VirtualBox, VMware, etc. kind of "just work". It is not the likely case that someone does a `vagrant up` and expects Docker, at this point.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.