[docs-ci] isntalling minimal fonts on the linux docker image

This merge request installs minimal fonts (dejavu-sans-fonts) on the linux docker image so documentations specifically graphviz renders properly.

merged

mentioned in commit dd9637be

mentioned in merge request !67 (closed)

mentioned in issue beat/docs#12 (closed)

Thx for identifying this issue - shouldn't we be caching the yum package downloaded?

It's just 3MB

OK then.