Trying to add a direct path to files doesn't work.
Dunno what's wrong here, but I do add a files direct path to /etc/ssl into a docker container and works as expected.
I think It's related to miniflux and have my self-signed certificate in its truststore to communicate with wallabag (inter-docker communication).
I can't give you a snipped of my compose but will gladly edited my comment when home.