gentoo docker image is fixed upstream
see https://github.com/gentoo/gentoo-docker-images/issues/69

this might break again if the upstream gentoo/portage image gets fixed

This is required to add svg support to mat2

Thanks, @poncho!

Closes #1

Due to a regression, gdk-pixbuff is currently broken in testing.
Accordingly, use stable. At the same time, bubblewrap is only available
in testing, accordingly, pull it from there.

Thanks to Poncho for debugging!

Signed-off-by: georg <georg@riseup.net>

Relates #1