Docker provider lacks support for exporting boxes

[mcandre]

When I try to export my Docker-provided Vagrant box, I get an error:

The "package" command is not supported with the Docker provider.
If you'd like to commit or push your Docker container, please SSH
into the host VM (if there is one), and run `docker commit` and
so on manually.

Fortunately, this is easy to implement with docker export and docker build, using tarballs. Can we please implement vagrant package for Docker provider, so that users can customize and publish Docker-provided Vagrant boxes?