Build and attack a local code runner

run_untrusted is the lesson's four rules turned into docker run flags and about 100 lines of host code. Code and inputs go in on stdin, so nothing from the host is mounted or inherited. Two limits can't be left to Docker: the wall-clock timeout and the stdout cap are enforced by the runner while it reads the container's output. The decision that matters most is the finally: killing the docker client on a timeout does not stop the container, so the runner always runs docker rm -f.