Dockerized Django: how to manage sql scripts in migrations?

Fairly new to Docker, I am trying to add the execution of a custom sql script (triggers and functions) to Django's migration process and I am starting to feel a bit lost. Overall, What I am trying to achieve follows this pretty clear tutorial. In this tutorial, migrations are achieved by the execution of an entry point script. In the Dockerfile:

# run entrypoint.sh
ENTRYPOINT ["/usr/src/my_app/entrypoint.sh"]

Here is the entrypoint.sh:

#!/bin/sh

if [ "$DATABASE" = "postgres" ]
then
    echo "Waiting for postgres..."

    while ! nc -z $SQL_HOST $SQL_PORT; do
      sleep 0.1
    done

    echo "PostgreSQL started"
fi

# tried several with and without combinations
python manage.py flush --no-input 
python manage.py makemigrations my_app 
python manage.py migrate

exec "$@"

So far so good. Turning to the question of integrating the execution of custom sql scripts in the migration process, most articles I read (this one for instance) recommend to create an empty migration to add the execution of sql statements. Here is what I have in my_app/migrations/0001_initial_data.py

import os
from django.db import migrations, connection

def load_data_from_sql(filename):
    file_path = os.path.join(os.path.dirname(__file__), '../sql/', filename)
    sql_statement = open(file_path).read()
    with connection.cursor() as cursor:
        cursor.execute(sql_statement)

class Migration(migrations.Migration):
    dependencies = [
        ('my_app', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(load_data_from_sql('my_app_base.sql'))
    ]

As stated by dependencies, this step depends on the initial one (0001_initial.py):

from django.conf import settings
from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    initial = True

    dependencies = [
        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
    ]

    operations = [
        migrations.CreateModel(
            name='Unpayed',
            fields=[
etc etc 

[The Issue] However, even when I try to manually migrate (docker-compose exec web python manage.py makemigrations my_app), I get the following error because the db in the postgresql container is empty:

File "/usr/src/my_app/my_app/migrations/0001_initial_data.py", line 21, in Migration
    migrations.RunPython(load_data_from_sql('my_app_base.sql'))
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 82, in _execute
....
    return self.cursor.execute(sql)
    django.db.utils.ProgrammingError: relation "auth_user" does not exist

[What I do not understand] However, when I log in the container, remove 0001_initial_data.py and run ./entrypoint.sh, everything works like a charm and tables are created. I can add 0001_initial_data.py manually later on, run entrypoint.sh angain and have my functions. Same when I remove this file before running docker-compose up -d --build: tables are created.

I feel like I am missing some obvious way and easier around trying integrate sql script migrations in this canonical way. All I need is this script to be run after 0001_initial migration is over. How would you do it?

[edit] docker-compose.yml:

version: '3.7'

services:
  web:
    build: 
      context: ./my_app
      dockerfile: Dockerfile
    command: python /usr/src/my_app/manage.py runserver 0.0.0.0:8000
    volumes:
      - ./my_app/:/usr/src/my_app/
    ports:
      - 8000:8000
    environment:
      - SECRET_KEY='o@@xO=jrd=p0^17svmYpw!22-bnm3zz*%y(7=j+p*t%ei-4pi!'
      - SQL_ENGINE=django.db.backends.postgresql
      - SQL_DATABASE=postgres
      - SQL_USER=postgres
      - SQL_PASSWORD=N0tTh3D3favlTpAssw0rd
      - SQL_HOST=db
      - SQL_PORT=5432
    depends_on:
      - db
  db:
    image: postgres:10.5-alpine
    volumes:
      - postgres_data:/var/lib/postgresql/data/ 

volumes:
  postgres_data:

django:2.2 python:3.7