Weird, I thought that was commented. tl;dr you can remove that.
It’s there because the book builds via an automated script that ssh’es into the Docker container running the example app, so when the book says “edit this file to change this” the script I use to maintain the book actually does that.
In my case, the entire thing is run via Docker Compose, so port 22 doesn’t conflict with my computer since the port in that case is inside the virtual network set up by Docker.
If none of that is interesting or makes sense, you can definitely ignore and just remove that bit from the Dockerfile.