It looks like jeedom doesn't create its database automatically. It expects you to first create a database, and set up a user with access to it.
So you need to use the mariadb docker addon to create it. But keep in mind that the db and user creation only happens during first start. Once the databases are set up, we no longer touch it, because we don't want to break your stuff. If you want to start over, you need to uninstall the mariadb addon and tell LE to remove user data as well. If you changed the data folder location in the addon settings, you'll need to manually delete that folder.
Then you can install mariadb addon again. It will open the settings page, allowing you to enter the user and database details. Once that's set up, then you can enter that info into jeedom.
In jeedom, use the libreelec lan ip as the mariadb address (localhost refers to inside the container).