Today, the issue showed up again, this time with my debugging code showing me the logfiles before shuttiing down the container:
"AH00170: caught SIGWINCH, shutting down gracefully"
What is SIGWINCH? It is a signal for "window change", aka, I resized the terminal window. Ok... that makes some sense that I would observe this occasionally, but not reproducibly, and resizing a terminal is certainly not something I had expected as the cause. But... why?