Is it fair to presume the docker person is going to be waiting a lot more for tests to run due to poor io performance?
If you're on a Linux laptop a Dockerized app has effectively no performance decrease.
If you're on a Mac, a VM and Docker does impact raw IO. My anecdotal experience developing like this every day is that it does not cause noticeable waiting.
There are other downsides to Docker-based development, around tool complexity and stability. In my opinion this is no worse than tools like Homebrew or rbenv.
There are also productivity upsides.