Most of the space is occupied by docker volume as you can see from your output:
12G ./volumes
Docker volumes are used to persist data for docker container and to share data between containers, and they are independent of the container’s lifecycle. So removing image/container will not free the disk space they occupied. Please refer to their official docs for more details.
If you're using latest version of docker, you can find volume related commands docs for more details(list/remove/create volumes e.g), for older version of docker, you can refer to this script on github for how to clean up volumes.
Hope this could be helpful:-)
