Access forbidden to Django resource when accessing through Node.js frontend - node.js

Question

I cloned a Django+Node.js open-source project, the goal of which is to upload and annotate text documents, and save the annotations in a Postgres db. This project has stack files for docker-compose, both for Django dev and production setups. Both these stack files work completely fine out of the box, with a Postgres database.
Now I would like to upload this project to Google Cloud - as my first ever containerized application. As a first step, I simply want to move the persistent storage to Cloud SQL instead of the included Postgres image in the stack file. My stack-file (Django dev) looks as follows
version: "3.7"
services:
backend:
image: python:3.6
volumes:
- .:/src
- venv:/src/venv
command: ["/src/app/tools/dev-django.sh", "0.0.0.0:8000"]
environment:
ADMIN_USERNAME: "admin"
ADMIN_PASSWORD: "${DJANGO_ADMIN_PASSWORD}"
ADMIN_EMAIL: "admin#example.com"
# DATABASE_URL: "postgres://doccano:doccano#postgres:5432/doccano?sslmode=disable"
DATABASE_URL: "postgres://${CLOUDSQL_USER}:${CLOUDSQL_PASSWORD}#sql_proxy:5432/postgres?sslmode=disable"
ALLOW_SIGNUP: "False"
DEBUG: "True"
ports:
- 8000:8000
depends_on:
- sql_proxy
networks:
- network-overall
frontend:
image: node:13.7.0
command: ["/src/frontend/dev-nuxt.sh"]
volumes:
- .:/src
- node_modules:/src/frontend/node_modules
ports:
- 3000:3000
depends_on:
- backend
networks:
- network-overall
sql_proxy:
image: gcr.io/cloudsql-docker/gce-proxy:1.16
command:
- "/cloud_sql_proxy"
- "-dir=/cloudsql"
- "-instances=${CLOUDSQL_CONNECTION_NAME}=tcp:0.0.0.0:5432"
- "-credential_file=/root/keys/keyfile.json"
volumes:
- ${GCP_KEY_PATH}:/root/keys/keyfile.json:ro
- cloudsql:/cloudsql
networks:
- network-overall
volumes:
node_modules:
venv:
cloudsql:
networks:
network-overall:
I have a bunch of models, e.g. project in the Django backend, which I can view, modify, add and delete using Django admin interface, but while trying to access them through Node.js views I get a 403 Forbidden error. This is the case of all my Django models.
For reference, in the above stack file, I have listed the only difference from the originally cloned Docker-compose stack file, where the DATABASE_URL used to point to a local Postgres Docker image, as follows
postgres:
image: postgres:12.0-alpine
volumes:
- postgres_data:/var/lib/postgresql/data/
environment:
POSTGRES_USER: "doccano"
POSTGRES_PASSWORD: "${POSTGRES_PASSWORD}"
POSTGRES_DB: "doccano"
networks:
- network-backend
To check if my GCP keys are correct, I tried to deploy the Cloud SQL Proxy container alone and interact with it (add, remove and update rows in included tables), and that was possible. However, the fact that I can use the Django admin interface successfully in the deployed Docker-compose stack should already prove that things are ok with the Cloud SQL proxy.
I'm not an experienced Node.js developer by any means, and have a little experience with Django and Django admin. My intention behind using a Docker-compose setup was that I will not have to bother with the intricacies of js views, and only have to deal with the Python business logic.