Docker containers are for the final version of a production ready piece of software.
Docker containers work best when your web app is built with scalability in mind, when you intend your app to end up in a https://kubernetes.io/ like environment.
In Docker, instead of running recipes to provision a server that, at the end of a sometimes very long process, also includes your WordPress code somewhow, you instead drop a git looking binary blob into your target and you are done. It's faster! Faster for ... production deployments ...
My experience with developing PHP on Docker is that it sucks. File mapping does not work the same way. The whole save then refresh the browser to see changes workflow doesn't work very well in Docker.
The Docker development flow is better suited for developers that recompile and/or restart their web server every time they save their files (Think: Typescript, Node.js, Python WSGI, Java ... )
Furthermore, Trellis is not deploying Vagrant into production? You can certainly jiggle your settings and end up with Docker there if you need that kind of scalability?
Things are improving for PHP on Docker though. If you want to play around with Docker check out: https://phpdocker.io/
I'm not against it, but there is certainly a lot of misunderstanding circulating for who or what Dockers was built for, where PHP Developers are last on the list.