Under this folder are .conf configuration files included by the Supervisor configuration file "/etc/supervisor/supervisord.conf".
Besides the .conf files under this folder, .conf files from folder ../service.d or ../task.d will be copied to here automatically when starting a Docker container.
If there are two .conf files of the same name there both under this folder and under folder ../service.d or ../task.d, the latter will be used. You may check file /usr/local/boot/supervisor.sh in the image and see how that happens.