Postgres in Docker persistent data - postgresql

Question

I'm running postgres inside a docker container to limit the amount of system resources it has access to. I'm having some trouble understanding how to make the data persistent. I've read the following articles:
https://www.andreagrandi.it/2015/02/21/how-to-create-a-docker-image-for-postgresql-and-persist-data/
http://container42.com/2013/12/16/persistent-volumes-with-docker-container-as-volume-pattern/
Which suggest using a data only container, and then having my postgres container link to it. What I'm failing to understand is; what's the advantage to this? As far as I can tell, if for some reason the docker-machine shut down (for example; moving it to a different physical machine), the data only container stops running, and all of it's contents are lost? I've tried creating a volume in the postgres container, but it doesn't actually seem to save anything to the disk.
Here's my docker file. What am I doing wrong?
FROM ubuntu
MAINTAINER Andrew Broadbent <andrew.broadbent#manchester.ac.uk>
# Add the PostgreSQL PGP key to verify their Debian packages.
# It should be the same key as https://www.postgresql.org/media/keys/ACCC4CF8.asc
RUN apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
# Add PostgreSQL's repository. It contains the most recent stable release
# of PostgreSQL, ``9.3``.
RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ precise-pgdg main" > /etc/apt/sources.list.d/pgdg.list
# Install ``python-software-properties``, ``software-properties-common`` and PostgreSQL 9.3
# There are some warnings (in red) that show up during the build. You can hide
# them by prefixing each apt-get statement with DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y python-software-properties software-properties-common postgresql-9.3 postgresql-client-9.3 postgresql-contrib-9.3
# Note: The official Debian and Ubuntu images automatically ``apt-get clean``
# after each ``apt-get``
# Run the rest of the commands as the ``postgres`` user created by the ``postgres-9.3`` package when it was ``apt-get installed``
USER postgres
# Create a PostgreSQL role named ``docker`` with ``docker`` as the password and
# then create a database `docker` owned by the ``docker`` role.
# Note: here we use ``&&\`` to run commands one after the other - the ``\``
# allows the RUN command to span multiple lines.
RUN /etc/init.d/postgresql start &&\
psql --command "CREATE USER docker WITH SUPERUSER PASSWORD 'docker';" &&\
createdb -O docker docker
# Complete configuration
USER root
RUN echo "host all all 0.0.0.0/0 md5" >> /etc/postgresql/9.3/main/pg_hba.conf
RUN echo "listen_addresses='*'" >> /etc/postgresql/9.3/main/postgresql.conf
# Expose the PostgreSQL port
EXPOSE 5432
# Add VOLUMEs to allow backup of config, logs and databases
RUN mkdir -p /var/run/postgresql && chown -R postgres /var/run/postgresql
VOLUME ["/etc/postgresql", "/var/log/postgresql", "/var/lib/postgresql"]
# Set the default command to run when starting the container
USER postgres
CMD ["/usr/lib/postgresql/9.3/bin/postgres", "-D", "/var/lib/postgresql/9.3/main", "-c", "config_file=/etc/postgresql/9.3/main/postgresql.conf"]

Answer 1

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