I have a project structure like this

project-root/
    back/
        Dockerfile
   docker-compose.yaml
    .env

Where docker-compose.yaml has

services:
  postgres:
    image: postgres:latest
    restart: always
    environment:
      POSTGRES_USER: ${POSTGRES_USER}
      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
      POSTGRES_DB: ${POSTGRES_DB}
    ports:
      - ${POSTGRES_PORT}:${POSTGRES_PORT}
    volumes:
      - ./data:/var/lib/posgresql/data      

  flask_app:
    network_mode: "host"
    build:
      context: ./back
      dockerfile: Dockerfile
    ports:
      - ${FLASK_PORT}:${FLASK_PORT}
    environment:
      FLASK_ENV: "development"
    depends_on:
      - postgres
    volumes:
      - ./back:/app

And my Dockerfile

FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .

RUN pip install --no-cache-dir -r requirements.txt

COPY . .

EXPOSE 5000

ENV FLASK_APP=main.py

CMD if [ "$FLASK_ENV" = "development" ]; then flask --debug run; else flask run; fi

and .env

POSTGRES_USER=postgres
POSTGRES_PASSWORD=password
POSTGRES_HOST=localhost
POSTGRES_PORT=5432
FLASK_PORT=5000

But my flask app is unable to connect to postgres with the following error:

psycopg2.OperationalError: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: No such file or directory
Is the server running locally and accepting connections on that socket?

Regular python scripts ran from my host like seed scripts and integration tests are able to connect to postgres with no problem. And when I move everything under /back the flask app within docker can connect as well. But I would like docker-compose.yaml
to live in the project root so I can build out a frontend and connect all the pieces together.

There must be some docker compose networking piece that I haven’t been able to figure out.

Any input is appreciated. Thanks!