Dockerfile: Include python3-tk for FATtools

Dockerfile: Include python3-tk for FATtools

In some cases our tests for exFAT don't run because we fail to be able
to create the underlying image. This is in turn because while creation
of the image succeeds, it seems that some way of how we invoke FATtools
wants to import tkinter, that fails and so the test stops there. Having
tkinter available (and then presumably a fallback to non-GUI because
it's not available) leads to the tests running as expected.

Reviewed-by: Marek Vasut <marek.vasut@mailbox.org>
Signed-off-by: Tom Rini <trini@konsulko.com>