nginx: [emerg] host not found in upstream when dockerizing a django/react project - django

Question

I'm trying to dockerize a django/react project but i'm running into this error when running docker-compose up.
i don't understand where the error come from.
i'm new to docker.
my goal is to deploy the frontend and the backend separately in one server.
nginx: [emerg] host not found in upstream
[emerg] 1#1: host not found in upstream "backend:8000" in /etc/nginx/conf.d/default.conf:2
nginx_1 | nginx: [emerg] host not found in upstream "backend:8000" in /etc/nginx/conf.d/default.conf:2
here is my code
django's Dockerfile
ENV DJANGO_SECRET_KEY $DJANGO_SECRET_KEY
ENV DJANGO_CORS_ORIGIN_WHITELIST $DJANGO_CORS_ORIGIN_WHITELIST
RUN mkdir /backend
WORKDIR /backend
COPY requirements.txt /backend/
EXPOSE 8000
RUN pip install -r requirements.txt
COPY . /backend/
RUN python manage.py makemigrations
RUN python manage.py migrate
react's Dockerfile
FROM node
USER root
WORKDIR /frontend
COPY . /frontend
ARG API_URL
ENV REACT_APP_HOST_IP_ADDRESS $API_URL
RUN yarn
RUN yarn config set ignore-engines true
RUN yarn build
server's config
upstream api {
server backend:8000;
}
server {
listen 8080;
location /api/ {
proxy_pass http://api$request_uri;
}
# ignore cache frontend
location ~* (service-worker\.js)$ {
add_header 'Cache-Control' 'no-store, no-cache, must-revalidate, proxy-revalidate, max-age=0';
expires off;
proxy_no_cache 1;
}
location / {
root /var/www/frontend;
try_files $uri $uri/ /index.html;
}
}
docker-copose.yml
version: '3'
services:
backend:
build:
context: ./ruelle_backend
args:
DJANGO_ALLOWED_HOSTS: http://ruelle-online.fr
DJANGO_SECRET_KEY: anis1807
DJANGO_CORS_ORIGIN_WHITELIST: http://ruelle-online.fr
command: gunicorn ivan_backend.wsgi --bind 0.0.0.0:8000
ports:
- "8000:8000"
frontend:
build:
context: ./ruelle_frontend
args:
API_URL: http://ruelle-online.fr
volumes:
- build_folder:/frontend/build
nginx:
image: nginx:latest
ports:
- 80:8080
volumes:
- ./webserver/nginx-proxy.conf:/etc/nginx/conf.d/default.conf:ro
- build_folder:/var/www/frontend
depends_on:
- backend
- frontend
volumes:
build_folder: