Postgres in Docker persistent data
This one answers your question about data container:docker mounting volumes on host
Regarding to your dockerfile, I would suggest you either:
1) use data container pattern
2) mount the volume to host machine by specifying: docker run -v [host-path]:[container-path] ..., so that data will be kept at one place in your host and will not be lost after the container is removed.
Ref: https://docs.docker.com/engine/tutorials/dockervolumes/#/mount-a-host-directory-as-a-data-volume