4.2G in /var/lib/docker/overlay2 on HA Pi server

Running HA in docker on a 8GB Pi 3B.
There is 4.2G in /var/lib/docker/overlay2
Any way to figure out why and if I can free space because I get this report from docker system df

TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              4                   3                   1.938GB             852MB (43%)
Containers          8                   3                   235.3kB             223.1kB (94%)
Local Volumes       24                  12                  0B                  0B
Build Cache         0                   0                   0B                  0B

and not much reclaimable:

root@pi:/home/daz# docker container prune
WARNING! This will remove all stopped containers.
Are you sure you want to continue? [y/N] y
Deleted Containers:

Total reclaimed space: 223.1kB

not much danlging around either:

root@pi:/home/daz# docker image ls -f dangling=true
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

and in desperation:

root@pi:/home/daz# docker system prune
WARNING! This will remove:
        - all stopped containers
        - all networks not used by at least one container
        - all dangling images
        - all dangling build cache
Are you sure you want to continue? [y/N] y
Total reclaimed space: 0B

Still 4.3GB

root@pi:/home/daz# sudo du -hs  /var/lib/docker/overlay2
4.2G    /var/lib/docker/overlay2


just entered linux single user runlevel mode (stops all services not needed) , then rm -rf /var/lib/docker (YUP!), then enter multiuser runlevel and with docker restarting, I started my docker containers , and voila! all good. 3GB free.

That is fairly drastic.