user accounts in docker group

[L L]

The banner at the top of https://wiki.debian.org/Docker says:

"The Docker daemon has setUID root, and by design allows easy access as root to the host filesystem. This makes it trivial for a malicious user to read and alter sensitive system files, or for a careless user to allow a malicious containerized app to do so. Access to Docker commands effectively grants full root power."

I'm trying to test this. I put my own user account in the docker group (and can execute docker commands with it).

Then I tried to see if I can use Docker to write a file to a root-owned directory without using sudo or su. I used these commands:
docker run debian -dit /bin/bash        #start a container
docker cp /home/me/some-file container-id:/some-file    #put a file into the container
docker cp container-id:/some-file /etc/some-file        #copy the file from the container into somewhere I shouldn't be able to write to

I got:
open /etc/some-file: permission denied

Is the wiki out of date and it's completely safe to have user accounts in the docker group?
Is the wiki correct but I'm exploiting group membership wrong?

[Yongun Seong]

I would try something like

docker run -v /:/host -it debian

Which I expect would give you root access to the "host" system inside
the container, under /host.

[Alex Mestiashvili]

Docker is insecure by design. If a user has access to the docker socket
(belongs to the docker group) he has root access to the host system.

See example here:
https://www.youtube.com/watch?v=CB9Aa6QeRaI