When I start the supervisor instead of using CMD, I don't have the error anymore, but the supervisor exit with code 0.
RUN service supervisor start
# RUN rm -f /var/run/supervisor.sock
# CMD ["supervisord", "-c", "/etc/supervisor/conf.d/supervisord.conf"]
And I don't have any other information logged to help understand how to solve this.
I have read on the web that it can have something to do with the user, but I have set user=root in the supervisor configuration file, so it should work no ?