Ops. I found the reason why this happens.
If my Dockerfile I have something like the following:
cd "$flarumDir"
if [ "$flarumPerformInstall" = "yes" ] ; then
envsubst </.attach/flarum-unattended-install.yml >install-config.yml
php5 flarum install -f install-config.yml
else
envsubst </.attach/flarum-config.php >config.php
php5 flarum migrate
fi
So, when I create a container with flarumPerformInstall = "no" the installation phase is completely omitted, not only the initial database migration.
Maybe there should be a separate command to collect static files, like the one Django has?
For now, I think I'll populate assets on my own. Are where anything else but 'fonts/' what I should copy?