I've started trying to understand Docker. However, it would appear that Docker can only "deploy containers" to AWS and MS Azure. We deploy Wordpress sites to Kinsta.
With regard to Wordpress and Kinsta, is Docker useful at all?
With regard to Wordpress and Kinsta, is Docker useful at all?
No.
Kinsta appears to offer fully-managed WordPress hosting, or "software as a service". You get to use WordPress via their administrative console and hosting tools, but you personally do not directly install WordPress or the underlying database.
Docker is a packaging and distribution mechanism for software. It's certainly common enough to install and deploy WordPress via Docker, but this isn't something you'd need to do with a SaaS service like Kinsta: they've already installed and deployed WordPress for you.