--output-dir with fully qualified path doesn't work

I’m not sure, but maybe I’m just too stupid to use Zola correctly.
i am using zola 0.16.1 (flatpak) under Debian stable in a fish shell.
The goal was to copy the zola documentation to my local webserver for easy access offline.
So in the docs directory I can without problems
zola build --output-dir zoladoc --base-url 'http://localhost/zola/'
without any problems.
But when I specify the destination directory fully qualified, the page is not created.
zola build --output-dir /var/tmp/zola/doc --base-url 'http://localhost/zola/'
No error is displayed either.
And now as I type it, it strikes me. Probably zola doesn’t have enough permissions via flatpak.
But then I would still expect an error message.
I’ll investigate further.