Docker password authentication failed for user "postgres" - postgresql

Question

I'm writing a docker-compose file to launch some services. But the db service is a trouble maker, I always get this error:
FATAL: password authentication failed for user "postgres"
DETAIL: Password does not match for user "postgres".
Connection matched pg_hba.conf line 95: "host all all all md5"
I've read a lot of threads, and I've correctly set the POSTGRES_USER and POSTGRES_PASSWORD. I have also remove the previous volumes and container to force postgresql to re-init the password. But I can't figure out why it's still not working.
So what is the correct way to force the re-initialization of the postgresql image. So I would be able to connect to my database.
I've seen that this error: Connection matched pg_hba.conf line 95: "host all all all md5", and I've heard about the postgres conf file. But it's an official container it's supposed to work, isn't it ?
version: '3'
services:
poll:
build: poll
container_name: "poll"
ports:
- "5000:80"
networks:
- poll-tier
environment:
- REDIS_HOST=redis
depends_on:
- redis
worker:
build: worker
container_name: "worker"
networks:
- back-tier
environment:
- REDIS_HOST=redis
- POSTGRES_HOST=db
- POSTGRES_DB=postgres
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=root
depends_on:
- redis
- db
redis:
image: "redis:alpine"
container_name: "redis"
networks:
- poll-tier
- back-tier
result:
build: result
container_name: "result"
ports:
- "5001:80"
environment:
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=root
- POSTGRES_HOST=db
- RESULT_PORT=80
networks:
- result-tier
depends_on:
- db
db:
image: "postgres:alpine"
container_name: "db"
restart: always
networks:
- back-tier
- result-tier
environment:
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=root
- POSTGRES_DB=postgres
volumes:
db-data:
driver: local
networks:
poll-tier: {}
back-tier: {}
result-tier: {}
I'm expected to get the db connected, and not password authentication failed for user "postgres".

Answer 1

Make sure your APPs (not the database container) are actually using the POSTGRES_USER and POSTGRES_PASSWORD variables. I suspect they are looking for something like DB_USER or similar and so aren't getting the right values in.
By default, every PostgreSQL database driver and admin tool defaults to the postgres user. This may explain why the error message complains about postgres even if the environment variable isn't being used.
A good way to verify is to change all references to the database user in the docker-compose file to something like postgres2. I suspect you'll still see apps complaining that password auth failed for postgres.

Answer 2

In my case, it was caused by postgres.exe service on Windows 10 running in the background. When I stopped the service, uninstalled PostgreSQL 12 from Windows, and restarted, I could finally connect to Postgres Docker container.