We run linux machines with hundreds of user accounts, it's safe. Why would you make any important files world-writable?
The right question is whether I have made any important files world-writable.
And the answer is “I don't know.”
So, containers.
And I run it with a special user id.
Now, does that machine have any important files that are world-writable? How sure are you? Probably less sure than for that machine with hundreds of users...
Yes, this is indeed the answer. Create a fake root. Create a user. Chmod and chgrp to restrict it to that fake root. ln /bin if you need to. Let it run wild in its own crib.