Add custom config location to Docker Postgres image preserving its access parameters - postgresql

Question

I have written a Dockerfile like this:
FROM postgres:11.2-alpine
ADD ./db/postgresql.conf /etc/postgresql/postgresql.conf
CMD ["-c", "config_file=/etc/postgresql/postgresql.conf"]
It just adds custom config location to a generic Postgres image.
Now I have the following docker-compose service description
db:
build:
context: .
dockerfile: ./db/Dockerfile
environment:
POSTGRES_PASSWORD passwordhere
POSTGRES_USER: user
POSTGRES_DB: db_name
ports:
- 5432:5432
volumes:
- ./run/db-data:/var/lib/db/data
The problem is I can no longer remotely connect to DB using these credentials if I add this Config option. Without that CMD line it works just fine.
If I prepend "postgres" in CMD it has the same effect due to the underlying script prepending it itself.

Answer 1

Provided all the files are where they need to be, I believe the only problem with your setup is that you've omitted an actual executable from the CMD -- specifying just the option. You need to actually run postgres:
CMD ["postgres", "-c", "config_file=/etc/postgresql/postgresql.conf"]
That should work!
EDIT in response to OP's first comment below
First, I did confirm that behavior doesn't change whether "postgres" is in the CMD or not. It's exactly as you said. Onward!
Then I thought there must be a problem with the particular postgresql.conf in use. If we could just figure out what the default file is.. turns out we can!
How to get the existing postgres.conf out of the postgres image
1. Create docker-compose.yml with the following contents:
version: "3"
services:
db:
image: postgres:11.2-alpine
environment:
- POSTGRES_PASSWORD=passwordhere
- POSTGRES_USER=user
- POSTGRES_DB=db_name
ports:
- 5432:5432
volumes:
- ./run/db-data:/var/lib/db/data
2. Spin up the service using
$ docker-compose run --rm --name=postgres db
3. In another terminal get the location of the file used in this release:
$ docker exec -it postgres psql --dbname=db_name --username=user --command="SHOW config_file"
config_file
------------------------------------------
/var/lib/postgresql/data/postgresql.conf
(1 row)
4. View the contents of default postgresql.conf
$ docker exec -it postgres cat /var/lib/postgresql/data/postgresql.conf
5. Replace local config file
Now all we have to do is replace the local config file ./db/postgresql.conf with the contents of the known-working-state config and modify it as necessary.
Database objects are only created once!
Database objects are only created once by the postgres container (source). So when developing the database parameters we have to remove them to make sure we're in a clean state.
Here's a nuclear (be careful!) option to
(1) remove all exited Docker containers, and then
(2) remove all Docker volumes not attached to containers:
$ docker rm $(docker ps -a -q) -f && docker volume prune -f
So now we can be sure to start from a clean state!
Final setup
Let's bring our Dockerfile back into the picture (just like you have in the question).
docker-compose.yml
version: "3"
services:
db:
build:
context: .
dockerfile: ./db/Dockerfile
environment:
- POSTGRES_PASSWORD=passwordhere
- POSTGRES_USER=user
- POSTGRES_DB=db_name
ports:
- 5432:5432
volumes:
- ./run/db-data:/var/lib/db/data
Connect to the db
Now all we have to do is build from a clean state.
# ensure all volumes are deleted (see above)
$ docker-compose build
$ docker-compose run --rm --name=postgres db
We can now (still) connect to the database:
$ docker exec -it postgres psql --dbname=db_name --username=user --command="SELECT COUNT(1) FROM pg_database WHERE datname='db_name'"
Finally, we can edit the postgres.conf from a known working state.

Answer 2

As per this other discussion, your CMD command only has arguments and is missing a command. Try:
CMD ["postgres", "-c", "config_file=/etc/postgresql/postgresql.conf"]