After CVAT is running and you have access to its login screen, you need to have an admin account to log in and access the admin panel.
You create an admin user from the command-line interface.
docker exec -it cvat bash -ic 'python3 ~/manage.py createsuperuser'
This command will as you for a username, email, and password.
After you log in, you can hover your username (in the top-right corner of the screen) and select "Admin page" to access the Django admin panel, where you can manage your CVAT site, manage users, groups, and more.