Extending docker official postgres image
If you define a new ENTRYPOINT in your Dockerfile it will override the inherited ENTRYPOINT. So in that case, Postgres will not be able to be initialized automatically (unless yo write the same ENTRYPOINT).
https://docs.docker.com/engine/reference/builder/#entrypoint
Also, the official postgres image let you add .sql/.sh files in the /docker-entrypoint-initdb.d folder, so they can be executed once the database is initialized.
Finally, if you don't want that Postgres remove your data, you can mount a volume between the /var/lib/postgresql/data folder and a local folder in each docker run ... command to persist them.