Get box info

ExitCode is how the box's main command ended, present once the runtime recorded it: its own code when it exited, or 128 + n when a signal ended it. Stopping a box signals that command, so a stop is recorded here too. Absence, not 0, is what says "not recorded": 0 is the exit code of every command that succeeded, so a reader has to tell a missing field from a zero one.