If you are using OpenLDAP, use this in your configuration.yml instead of the file authentication.
First, follow the guide here if you have not done so already.
In your configuration.yml, now replace the file/LDAP section with the below and fill in the details accordingly, remembering to replace domain with your domain details. If you are running the openldap container outside the docker network, you will have to replace openldap in the url section for the openldap container IP.
Source - https://www.authelia.com/docs/configuration/authentication/ldap.html#configuration