Phoenix Framework - docker image for development and deployment to Kubernetes
In my opinion you are on the right track. Including the code in the image is probably considered best practice. I have recently written an answer to another question describing some of the benefits.
In practice people are using all kinds of ways to serve the application to a container. It is possible to use attached volumes or have a Git in the container pull/update the code when deployed but I believe you would need some good reason (that I can't think of) for that being preferable.