localhost in Docker context means only the container itself will have permissions to write to the DB
As the LibreNMS docs/installation expects you to install in a Linux instance, localhost makes sense as no one else should be able to access the DB from outside
Because we are using containers, you will need to add % so that any IP/network can have access to the container
For security hardening, you can set the container to only accept IP from another container for eg. 'librenms'@'172.17.0.3'