Permission error when running 'bin/console cache:clear'

[simontol]

When I run 'bin/console cache:clear' I get the following error:
Serializer.php line 302:
User Warning: Directory /app/var/cache/prod/htmlpurifier/HTML not writable, please alter file permissions

Unfortunately this breaks the deploy pipeline.
We started getting this error since we upgraded from exercise/htmlpurifier-bundle 2.0 to 5.0 and Symfony from 5.4 to 6.4