-i (interactive) and -t (allocate a tty) allow the shell running
inside Docker to handle Ctrl-C (^C) and other shell commands, so you
can stop a command in the interactive process you ran it. However,
they only work if the containing shell (the one where you ran `docker
run`) is also interactive with a tty hooked up, so we test for that
first in both scripts that invoke `docker run`, using `test -t`.
--init passes signals from *outside* Docker down to its subprocesses,
so that cancellation from *another* context works for our Docker
images. This includes the Cancel button in GitHub Actions.
