Fix Docker "no space left on device"

Builds or `docker pull` fail with `no space left on device`, but `df -h /` looks fine.

Docker stores images, containers, volumes and build cache under `/var/lib/docker`. Layers and BuildKit cache add up fast.

See what is using space

docker system df -v

Prune safely

docker container prune -f
docker image prune -af
docker builder prune -af

Remove unused volumes (DESTRUCTIVE — data is lost)

docker volume prune -f