The answer is here:
"A common practice is to add users that need to run Docker containers on your host to the docker group. [...] What is not obvious right away is that this is basically the same as giving those users root access. You see, the Docker daemon runs as root and when you add users to the docker group, they get full access over the Docker daemon."
Correct. You are expected to map the in-container root to an unpriv user on the parent NS. If the mapping maps in-container root to a user with some privileges on the parent NS, there are some more complex rules applying to what you can and can't do. That's because there are still stuff like mount point locks, and setns restrictions.
@bortzmeyer @ParadeGrotesque @Keltounet
My answer was inaccurate. Even if mapped to an unpriv user, you have to care about mount point locks and setns restrictions and all of that "good" stuff, but things get even muddier when mapped to a user able to get some privileges and then, I'm not sure what happens.
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