What is the su password

Just downloaded the image “Microsoft Artifact Registry.
This is SQL running on Ubuntu.

Does anyone know the default superuser (root) password for Ubuntu?
Many thanks

mcr.microsoft.com/mssql/server:2019-latest

There are no passwords for users in containers. Why would you need it? What are you trying to do? You can run commands in containers as any user

you can pass --user root to your docker command
e.g. something like this:
docker exec -it --user root yourcontainer bash