

The problem is that while docker is running as root (0) the searxng process internally runs as a user searxng with id 977
So your filesystem outside of docker needs to have those files assigned the right ownership.
This thread has a more detailed discussion that you might find helpful. https://forums.truenas.com/t/how-to-install-searxng-app-docker-permissions-issue/14049/10
GliNet makes great openwrt based devices, they have their own more userfriendly front end, but allow power users to enable acess to the standard openwrt features and packages under the hood.