Fix swh-docs builds (publish and dev)

The docs publish and dev job are broken. The gist of it is [1]. The full stack trace is [2]. plantuml version: 1.2021.12 [3].

This needs some fixing.

• [1]

make[3]: Entering directory '/home/jenkins/workspace/DDOC/publish/swh-fuse/docs/images'
plantuml -Tpdf arch-container.puml
java.lang.ClassNotFoundException: org.apache.batik.apps.rasterizer.SVGConverter

• [2] https://jenkins.softwareheritage.org/view/all/job/DDOC/job/publish/1136/console https://jenkins.softwareheritage.org/view/swh-draft/job/DDOC/job/dev/6845/console

• [3] https://forge.softwareheritage.org/source/swh-jenkins-dockerfiles/browse/master/sphinx/Dockerfile$5

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Migrated from T3665 (view on Phabricator)