So without knowing much about you setup, I would say that there is a cronjob set to run in the background at 3am. You can enter the docker shell and check crontab and see what is scheduled to run. crontab -e to edit for the current user. crontab -l to check the list of configured tasks. Keep in mind that it is by user so any user may be the one with the cron configured, I suppose. It is also possible that the cron is set up on the docker host as well. So maybe try to check crontab on the host as well.
Hopefully, this is helpful. I am not an expert.