Selfhosted

Sure, you can just add a bind mount volume to mount a path from the host system to the container.

The only thing you'd have to make sure of is that whatever uid/gid the container is running as has permissions to access whatever cert files you're after (but if you're being bad and running as root, then congrats, that's not usually an issue!)

Edit: redacted docker-compose example of this:

  service-name:
    image: image-tag
    restart: unless-stopped
    container_name: service-name
    hostname: service-name
    command: '--cert /data/live/domain.com/cert.pem --key /data/live/domain.com/privkey.pem port ip:23'
    ports:
      - port-external:port-internal
    volumes:
      - /etc/letsencrypt:/data:ro

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.