Changes

scripts/docker-cleanup: add "docker system prune"

On build4 this freed up another 8.223GB when I just ran it:

  $ 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]

Change-Id: I98aa5324eaf7cf319625d85bc80d431b89ee50a7