How to run a SQL script in a multi-container app from a custom entrypoint.sh?

Question

This is not a "I need to run a SQL script right after the PostgreSQL container is built" question.

I have a FastAPI app with a dockerized PostgreSQL. My problem is: I want to populate the database upon app launch, but my tables are all generated by SQLAlchemy when I run uvicorn. So I need to be able to run the SQL with the INSERT statements right after uvicorn is executed.

To do that, I tried creating an entrypoint.sh file to run psql commands, but at the end of docker compose up I always get:

psql: command not found

I figured it's because Docker isn't inside the PostgreSQL container when it's executing entrypoint.sh, but rather inside the main app container.

entrypoint.sh:

#!/bin/bash

uvicorn backend.main:app --host 0.0.0.0 --port 8000 &

sleep 3

# Execute the populate.sql script
psql postgresql://postgres:postgres@db:5432/wikiprof
psql -a -f ./backend/populate.sql

Dockerfile:

FROM python:3.11
WORKDIR /wikiprof
COPY ./requirements.txt /wikiprof/requirements.txt
COPY ./backend/ /wikiprof/backend/
COPY ./frontend/ /wikiprof/frontend/
COPY ./entrypoint.sh /wikiprof/
RUN chmod +x /wikiprof/entrypoint.sh
RUN pip install --no-cache-dir --upgrade -r /wikiprof/requirements.txt
CMD ["/wikiprof/entrypoint.sh"]

compose.yml:

services:
  db:
    image: postgres
    ports:
      - "5434:5432"
    environment:
      - "POSTGRES_USER=postgres"
      - "POSTGRES_PASSWORD=postgres"
      - "POSTGRES_DB=wikiprof"
    volumes:
      - "/home/gabriel/wikiprof_bd:/var/lib/postgresql/data"
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres", "-d", "wikiprof"]
      interval: 3s
      timeout: 50s
      retries: 10

  app:
    build: .
    ports:
      - "8000:8000"
    depends_on:
      db:
        condition: "service_healthy"

Is there any way I get inside the db container from entrypoint.sh and run psql? The main goal is running the SQL INSERTs upon launch, with no manual command running.

Am I technically doing something wrong or shouldn't I be having this approach altogether?