Simplify cleanup even further

Following Stephen Kitt's excelent observation, it appears that -f flag
is not necessary.
However, after a bit further digging I've discovered that `docker system
prune` will provide us with all this functionality, so why reimplment ;)

For reference (if you're lazy):
$ docker system prune --help

Usage:	docker system prune [OPTIONS]

Remove unused data

Options:
  -a, --all             Remove all unused images not just dangling ones
      --filter filter   Provide filter values (e.g. 'label=<key>=<value>')
  -f, --force           Do not prompt for confirmation
      --volumes         Prune volumes

$ docker system prune --volumes
WARNING! This will remove:
  - all stopped containers
  - all networks not used by at least one container
  - all volumes not used by at least one container
  - all dangling images
  - all dangling build cache

Are you sure you want to continue? [y/N]

scripts/shared/cleanup.sh

@@ -20,36 +20,10 @@ function stop_local_registry {
     fi
 }
 
-function cleanup_containers {
-    local containers=$(docker ps -qf status=exited)
-    if [[ -n ${containers} ]]; then
-        echo "Cleaning containers..."
-        docker rm -f ${containers}
-    fi
-}
-
-function cleanup_images {
-    local dangling_images=$(docker images -qf dangling=true)
-    if [[ -n ${dangling_images} ]]; then
-        echo "Cleaning images..."
-        docker rmi -f ${dangling_images}
-    fi
-}
-
-function cleanup_volumes {
-    local dangling_volumes=$(docker volume ls -qf dangling=true)
-    if [[ -n ${dangling_volumes} ]]; then
-        echo "Cleaning volumes..."
-        docker volume rm -f ${dangling_volumes}
-    fi
-}
-
 
 ### Main ###
 
 run_parallel "{1..3}" delete_cluster
 stop_local_registry
-cleanup_containers
-cleanup_images
-cleanup_volumes
+docker system prune --volumes -f