Asciidoctor Icons

Having set up a README.adoc I noticed that fontawesome fonts do not work as I expected.
So, for example, icon:cube doesn’t appear.

it looks like gitlab doesn’t express the classes.

Is there a way around this?

My attributes:

:imagesdir: images
:experimental:
:unsafe:
:icons: font
:icon-set: fa
:!sectnums:
:version:
:doctype: article