Ok I was able to trigger an error when posting to the list in the mailmanweb.log:
ERROR 2018-12-22 22:12:01,813 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS. ERROR 2018-12-22 22:12:01,830 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS. ERROR 2018-12-22 22:12:03,243 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS. ERROR 2018-12-22 22:12:03,251 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS. ERROR 2018-12-22 22:12:03,350 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS. ERROR 2018-12-22 22:12:03,419 34 django.security.DisallowedHost Invalid HTTP_HOST header: 'mailman-web:8000'. You may need to add 'mailman-web' to ALLOWED_HOSTS.
Brian