make prune will remove all the currently unused volumes

[gluckzhang]

First of all, thanks for this nice work!

Not a bug though, in the Makefile, I see that docker volume prune -f is used. But maybe users only want to delete the ELK-related volumes. For example, I just accidentally removed all the volumes on my computer. Luckily there is no important data lost 😄

I would suggest adding a filter in this docker volume prune command. This will be done in a minute with a new PR.

Cheers!