Docker doesn't actually support secrets if you aren't running a swarm. Docker-compose secrets are just read-only bind mounts under the covers.
If you are not using swarm, the plain-text of the "secret" is stored on disk anyway. There is a benefit to sharing secrets in this way; however, as you avoid unintentionally leaking sensitive environment variables between containers.
In a compose file, this would be the same as using "secrets":
version: "3.7"
services:
example:
image: traefik:latest
volumes:
- type: bind
source: ./secret_file.txt
target: /var/secrets/secret_file.txt
read_only: true
Thoughts about the docker implementation in Unraid aside, you can add a file to your server and bind mount it through the UI or XML.
If you really want to add secrets, you will have to setup your own vault.