The toolbox command in CoreOS uses Fedora by default. If you'd rather it used Debian by default, you can add the following lines to .toolboxrc:
TOOLBOX_DOCKER_IMAGE=debian
TOOLBOX_DOCKER_TAG=jessie
When you next run toolbox, you should see it pull down the requested image.
$ toolbox
Pulling repository debian
835c4d274060: Download complete
511136ea3c5a: Download complete
16386e29a1f4: Download complete
Status: Downloaded newer image for debian:jessie
core-debian-jessie
Spawning container core-debian-jessie on /var/lib/toolbox/core-debian-jessie.
Press ^] three times within 1s to kill container.
root@myserver:~#
It's that simple.