Where is the Postgres username/password being created in this Dockerfile? - postgresql

Question

So I was following this tutorial:
https://realpython.com/blog/python/django-development-with-docker-compose-and-machine/
I have everything up and running, however theres a few things going on that I'm not able to follow or understand.
In the main Docker-Compose we have:
web:
restart: always
build: ./web
expose:
- "8000"
links:
- postgres:postgres
- redis:redis
volumes:
- /usr/src/app
- /usr/src/app/static
env_file: .env
command: /usr/local/bin/gunicorn docker_django.wsgi:application -w 2 -b :8000
postgres:
restart: always
image: postgres:latest
ports:
- "5432:5432"
volumes:
- pgdata:/var/lib/postgresql/data/
You will notice there is a env_file containing:
DB_NAME=postgres
DB_USER=postgres
DB_PASS=postgres
My question is when is the postgres user and password being set? If I run this docker-compose everything works, meaning the web app can pass credentials to the postgress database and establish a connection. I'm not able to follow however, where those credentials are being set in the first place.
I was assuming in the base postgres Dockerfile, there would be some instruction to set the database name, username and password. I do not it though. Here is a copy of the base postgres Dockerfile below.
# vim:set ft=dockerfile:
FROM debian:jessie
# explicitly set user/group IDs
RUN groupadd -r postgres --gid=999 && useradd -r -g postgres --uid=999 postgres
# grab gosu for easy step-down from root
ENV GOSU_VERSION 1.7
RUN set -x \
&& apt-get update && apt-get install -y --no-install-recommends ca-certificates wget && rm -rf /var/lib/apt/lists/* \
&& wget -O /usr/local/bin/gosu "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture)" \
&& wget -O /usr/local/bin/gosu.asc "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture).asc" \
&& export GNUPGHOME="$(mktemp -d)" \
&& gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 \
&& gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu \
&& rm -r "$GNUPGHOME" /usr/local/bin/gosu.asc \
&& chmod +x /usr/local/bin/gosu \
&& gosu nobody true \
&& apt-get purge -y --auto-remove ca-certificates wget
# make the "en_US.UTF-8" locale so postgres will be utf-8 enabled by default
RUN apt-get update && apt-get install -y locales && rm -rf /var/lib/apt/lists/* \
&& localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
ENV LANG en_US.utf8
RUN mkdir /docker-entrypoint-initdb.d
RUN apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
ENV PG_MAJOR 9.6
ENV PG_VERSION 9.6.1-1.pgdg80+1
RUN echo 'deb http://apt.postgresql.org/pub/repos/apt/ jessie-pgdg main' $PG_MAJOR > /etc/apt/sources.list.d/pgdg.list
RUN apt-get update \
&& apt-get install -y postgresql-common \
&& sed -ri 's/#(create_main_cluster) .*$/\1 = false/' /etc/postgresql-common/createcluster.conf \
&& apt-get install -y \
postgresql-$PG_MAJOR=$PG_VERSION \
postgresql-contrib-$PG_MAJOR=$PG_VERSION \
&& rm -rf /var/lib/apt/lists/*
# make the sample config easier to munge (and "correct by default")
RUN mv -v /usr/share/postgresql/$PG_MAJOR/postgresql.conf.sample /usr/share/postgresql/ \
&& ln -sv ../postgresql.conf.sample /usr/share/postgresql/$PG_MAJOR/ \
&& sed -ri "s!^#?(listen_addresses)\s*=\s*\S+.*!\1 = '*'!" /usr/share/postgresql/postgresql.conf.sample
RUN mkdir -p /var/run/postgresql && chown -R postgres /var/run/postgresql
ENV PATH /usr/lib/postgresql/$PG_MAJOR/bin:$PATH
ENV PGDATA /var/lib/postgresql/data
VOLUME /var/lib/postgresql/data
COPY docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
EXPOSE 5432
CMD ["postgres"]

Answer 1

Not sure which postgres image are you using.
If you look at the official postgres image for complete information. It allows user to specify the environment variables for the below ones and these can be easily overridden at run time.
POSTGRES_PASSWORD
POSTGRES_USER
PGDATA
POSTGRES_DB
POSTGRES_INITDB_ARGS
Environment variables can be overridden using below three methods depending on your case.
Run Image: If running docker image directly, use below to include environment variable in docker run with -e K=V. Please refer documentation for more details here
docker run -e POSTGRES_PASSWORD=secrect -e POSTGRES_USER=postgres <other options> image/name
Dockerfile: If you need to specify the environment variable in Dockerfile, specify as mentioned below. Please refer documentation for more details here
ENV POSTGRES_PASSWORD=secrect
ENV POSTGRES_USER=postgres
docker-compose: If you need to specify the environment variable in docker-compose.yml, specify as below. Please refer documentation for more details here
web:
environment:
- POSTGRES_PASSWORD=secrect
- POSTGRES_USER=postgres
Hope this is useful.

Answer 2

I think the postgres user and password is being set on entrypoint, like in line 23 on official image entrypoint.
https://github.com/docker-library/postgres/blob/e4942cb0f79b61024963dc0ac196375b26fa60dd/9.6/docker-entrypoint.sh
Can you check your entrypoint?

Answer 3

All you need is
docker run -e POSTGRES_PASSWORD=123456 postgres
In the same way you can set up the username like below:
docker run -e POSTGRES_USERNAME=xyz postgres

Answer 4

Are you using the -v mounted folder? If so, you need to remove the entire pgdata/ folder you created previously so that the postgres can re-create itself using the new environment variables.