that's exactly what i did. i wrote the blog post for anyone who wanted to do it from scratch, start to finish...but in my case i just used my existing channels instance / docker stack and added a second one to my compose file as the backend.
i'd just add a third container. if you screw up that container you can always trash it and start over...that's the whole point of docker, keeping everything separate like that.
nope, i use the default 8089 for the frontend so as to not confuse any of the apps. i use 8090 for the backend. you can't use the same port on the same machine for both containers. docker won't let you. it wouldn't have any way of knowing which one you wanted on any given request.