Most of the space was taken by docker logs of the running containers. Easiest solution was to recreate the containers that had long logs, which cleared the logs.

A permanent solution would be to set up rotation of the docker logs, but I remember looking into this earlier and it was not straightforward.

Ok, the following settings for each service in the docker-compose.yml file may have fixed this:

logging:
    driver: json-file
    options:
        max-size: 10m
        max-file: "20"
        compress: "true"