I am attempting to dockerize my flask + postgres app, but I am running into this problem (none of the answers in that question helped me). Basically I want the docker-compose file to create a database with a different name than postgres. I would like to keep all of my docker setup contained in the dockerfile, docker-compose, and .env file. Is there anyone who can help?

Dockerfile:

FROM python:3.12-slim

RUN apt-get -q -y update 
RUN apt-get install -y gcc

ENV USERNAME=app_user
ENV WORKING_DIR=/home/app_db

WORKDIR ${WORKING_DIR}

COPY app app
COPY requirements.txt .
COPY service_entrypoint.sh .

RUN groupadd ${USERNAME} && useradd -g ${USERNAME} ${USERNAME}

RUN chown -R ${USERNAME}:${USERNAME} ${WORKING_DIR}
RUN chmod -R u=rwx,g=rwx ${WORKING_DIR}

USER ${USERNAME}
ENV PATH "$PATH:/home/${USERNAME}/.local/bin"

RUN pip install --upgrade pip
RUN pip install -r requirements.txt

ENV FLASK_APP=app_db:serve

RUN chmod +x service_entrypoint.sh

EXPOSE 5000
RUN flask db init

ENTRYPOINT [ "./service_entrypoint.sh" ]

docker-compose.yml:

services:
  db:
    image: postgres
    container_name: db
    restart: always
    ports:
      - 5123:5432
    env_file:
      - path: ./.env    
    volumes:
      - ./pData:/var/lib/postgresql/data
    healthcheck:      
      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB} -t 1"]
      interval: 10s
      timeout: 10s
      retries: 10
      start_period: 10s
  app:
    container_name: app
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - 8008:5000    
    depends_on:
      db:
        condition: service_healthy    
    links: 
        - db

A .env file:

POSTGRES_DB=app_db
POSTGRES_USER=postgres 
POSTGRES_PASSWORD=password

I don’t know if this means anything, but I also get this message:

db        | PostgreSQL Database directory appears to contain a database; Skipping initialization

The error message i get (I am assuming that it is from the healthcheck) is FATAL: database "app_db" does not exist. As a last note, when I attach to the postgres image in a tty shell, there is only the database postgres and a couple of templates.