> The main advantage of docker, as far as my understanding goes, is more of a prebuilt system configuration thing

I think the main advantage is that it standardizes the interface around the application image/container. This allows powerful abstractions to be written once rather than requiring a bespoke implementation for each way that the application is structured. Imagine writing the equivalent of Kubernetes around some hacked-together allows-multiple-versions-of-a-dependency solution. It would be a nightmare. But because the Docker image/container interfaces are codified, you can build powerful logic around those boundaries without needing to understand what's inside those images/containers. Dynamically shifting load, recovering from failures, automatic deployments and scaling are all much easier when you don't have to worry about what language the application is written in or how the application is structured.