Moin,
the hard disk is almost full. As far as I see, there are 170 GB ununsed docker images:
renner@boulder:~$ docker system df
TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          6         6         2.594GB   178.2MB (6%)
Containers      6         6         170.1GB   0B (0%)
Local Volumes   5         5         729.9kB   0B (0%)
Build Cache     0         0         0B        0B
following the instructions i tried various commands:
$ docker image prune -a -f
$docker system prune
$ docker system prune --volumes
$ docker container prune 
But there are still unreclaimable.
$ docker container ls -a
only shows the running containers, only a few.
renner@boulder:~$ docker --version
Docker version 20.10.6, build 370c289
What did I miss?