What it's supposed to make easier is both sandboxing and reuse / deployment. For example, Docker + Traefik makes some tasks so incredibly easy and secure compared to running them on bare metal. Or if you need to spin up multiple instances, they can be created and destroyed in seconds. Without the container, this just isn't feasible.

The dockerfile uses MySQL because it works. If you want to know if the core service works with PostgreSQL, that's not really on the guy who wrote the dockerfile, that's on the application maintainer. Read the docs, do some testing, create your own container using its own PostgreSQL or connecting to an external database if that suits your needs better.

Once again the flexibility of bind mounts means you could often drop that external database right on top of the one in the container. That's the real beauty of Docker IMO, being able to slot the containers into your system seamlessly due to the mount system.

adapting can be a pita when the package is built around a really specific environment

That's the great thing about Docker, it lets you bring that really specific environment anywhere and in an incredibly lightweight manner compared to the old days of heavyweight VMs. I've even got Docker containers running on a Raspberry Pi B+ that otherwise is so old that it would be nearly impossible to install the libraries required to run modern software.