Actually, I came across another thread where you suggested increasing the "graceful_timeout" in gunicorn.cfg. In fact this didn't work for me, but I followed your reference to the documentation and tried increasing just "timeout" instead - and that worked!
It's pretty slow, but I can now get all my (very long) mailing list members via the REST API, which is great.
In your post, you mentioned that "You can add a new gunicorn.cfg next to your mailman.cfg and add the additional options." I tried creating such a file with my new "timeout" setting in /etc, but it had no effect. The only way I could get the setting to stick was by putting it in /usr/lib/python3.6/site-packages/mailman/config/gunicorn.cfg. Am I missing something obvious here? It would be nice to be able to configure this setting from outside the container.
Thanks for any advice. Stephen