contrib/docker/rootfs_prefix: Add MIT license header.

On 2018-09-06 Marc Fournier wrote:

Hello Florian !

Yes, sure, sorry to have omitted this! I'm fine with MIT. Glad this is
useful to others :-)

+1 for adding a default "fallback" licence to COPYING. In case this has
unexpected implications, let's just add the MIT licence to root_prefix/* so
Baruch can move forward with his project.

Cheers,
Marc