Note that unfortunately there is a conflict between VirtualBox and Docker on Windows hosts, preventing both from running concurrently. Windows-host users are encouraged to run all sandboxes with the same hypervisor, no mixing and matching without (yuck!) rebooting. Hyper-V VM's and Docker containers can probably coexist, though few Vagrant Cloud images support Hyper-V at the moment.
For Linux guests, offer Docker provider, so that more users focused on Linux can leverage more performant Vagrant boxes.
http://packer.io/docs/builders/docker.html
https://www.vagrantup.com/docs/docker/basics.html
Note that unfortunately there is a conflict between VirtualBox and Docker on Windows hosts, preventing both from running concurrently. Windows-host users are encouraged to run all sandboxes with the same hypervisor, no mixing and matching without (yuck!) rebooting. Hyper-V VM's and Docker containers can probably coexist, though few Vagrant Cloud images support Hyper-V at the moment.