A benchmark to see how much perfs you loose by virtualizing Postgres would have been nice.
I would say that there is just a little overhead as Postgres data volume is mounted inside Docker container. Running Postgres in docker container without volumes would have a bigger impact.
And Docker container is not a VM, so I don't like to use word Virtualization for containers :)
It would be interesting. But I would also note that Docker doesn't do virtualization. It uses e.g. mount namespaces to mount an existing Linux volume inside the container (http://man7.org/linux/man-pages/man7/namespaces.7.html).