Designed to be used as a base image, you must provide your own Dockerfile. A minimal one can contain just the FROM instruction.
You must provide a lmConf-1.js file and set the base domain name the server will listen to with the variable SSODOMAIN.
Use the docker build command:
docker build --build-arg SSODOMAIN=example.com -t yourname/lemonldap-ng:version .
Or use the arg option in your docker-compose.yml file (https://docs.docker.com/compose/compose-file/#/args).