Microservices are specifically made for a given context. It's a "style" of making a service, you can't turn something into one. Well that's a lie, the devs could, but no one is going to ask ops to chop up a big rails project into microservices, and its pretty much impossible without re-writing it. The devs basically just write the microservice and give it to you. Usually it's already in a docker container because it makes hooking it up to all the other ones a lot easier.
Docker isn't just a microservice thing though, it's more about putting each program(or a microservice) into it's own container. You could dockerize(containerize?) a wordpress install: container for mysql, one for the php app, one for nginx. Or do the same for a mail server, or IRC server.