i don't see how it's a design mistake, linux allows more footguns in general to not decrease utility. Allowing you to manually give root prompt access (with warnings!) to a non-root user is one of them.

you can also just not run docker as root and not add normal users to the docker group

> And containers were supposed to make things safer ...

No. Containers are a slight improvement over the .tar.gz software distribution method we had a few decades ago.

(And I mean "slight" literally - a Docker container is just a .tar.gz with a bundled bash script that runs in a chroot.)

Containers were never a security boundary