Where does disk memory leak when using Docker?

Question

Docker

Hello!
I can’t figure out what’s wrong, I have Docker installed on my Wirth machine, there are several containers in it. Slowly but surely, this thing eats up disk memory (all my 8GB), although nothing new is added to the containers.
1. First, I check how much space the containers take up: docker ps -s in total no more than 1.5GB, which is normal, I look for where the rest of the space is.
2. I use the ncdu utility and find that /var/lib/docker occupies 4.5 GB , it has this picture:
2.0GiB /devicemapper
1.3GiB /volumes
1.1GiB /containers
From here the question is, for starters, what is a devicemapper and how to deal with it?
Googled but couldn't figure it out.
I would be grateful for any input.
Thank you!