* Fix for sphinx-gallery 0.3.1.
Figure numbering have been changed to ignore matplotlib figure number.
* Use circleci/3.6 image.
This docker image uses Debian stretch and fixes the problems seen with apt-get update with Debian jessie.
* Install additional fonts.
Seems to be needed to convert doc/images/iris.svg.
* Another missed example using figure number 0.
[doc build]