docker-compose: postgres data not persisting

I have a main service in my docker-compose file that uses postgres's image and, though I seem to be successfully connecting to the database, the data that I'm writing to it is not being kept beyond the lifetime of the container (what I did is based on this tutorial).

Here's my docker-compose file:

main:
  build: .
  volumes:
    - .:/code
  links:
    - postgresdb
  command: python manage.py insert_into_database
  environment:
    - DEBUG=true


postgresdb:
  build: utils/sql/
  volumes_from:
    - postgresdbdata
  ports:
    - "5432"
  environment:
    - DEBUG=true


postgresdbdata:
  build: utils/sql/
  volumes:
    - /var/lib/postgresql
  command: true
  environment:
    - DEBUG=true

and here's the Dockerfile I'm using for the postgresdb and postgresdbdata services (which essentially creates the database and adds a user):

FROM postgres

ADD make-db.sh /docker-entrypoint-initdb.d/

How can I get the data to stay after the main service has finished running, in order to be able to use it in the future (such as when I call something like python manage.py retrieve_from_database)? Is /var/lib/postgresql even the right directory, and would boot2docker have access to it given that it's apparently limited to /Users/?

Thank you!