By installing the Web Map Client, you will be able to visualise the cityGML data in CesiumJS. More details here.
Install the Docker image of 3DcityDB-Web-Map-Client. The instructions are based on this post.
$ docker run -d --name 3dwebmap -p 8000:8000 \ -v /home/chaos/3dcitydata/:/var/www/data/ \ tumgis/3dcitydb-web-map