No, not everyone will move to Docker.
Docker is a lightweight virtualization solution (somewhat similar to VMware, but more lightweight / less demanding of the hardware) that isolates an app from the underlying OS. Apps can be packaged in a container that includes most of the software environment the app needs, such as programming libraries that the OS does not provide. The container runs in Docker instead of directly on the OS (Docker (software) - Wikipedia)
Future Roon versions will update to Microsoft’s .NET 10, and this has certain requirements for the OS regarding programming library versions that the OS provides. Certain older OSes (most notably QTS on QNAP NAS and DSM 6 on Synology NAS) do not provide these libraries at the necessary newer versions. However, they do provide solutions to run Docker containers. Therefore, Roon Labs will provide Roon Server in a Docker container (packaged together with these necessary libraries) for users who are stuck with such older OSes.
This is irrelevant to everyone else as explained in the announcement: