docker nginx django gunicorn 502 bad gateway - django

Question

i am a beginner in the docker I want deploy my django project on hostinger.com vps so I am using docker nginx and gunicorn for this I dockrized my django project a test it on the localhost every thing is good and my project is working by when i deploy it on the vps it show me 502 Bad gateway
i don't have any idea ?
docker-compoe.yml
version: "3.7"
services:
app:
build: './app'
container_name: 'app'
restart: 'always'
expose:
- '8000'
# ports:
# - "8000:8000"
environment:
- "MARIADB_DATABASE=django_app"
- "MARIADB_USER=peshgaman"
- "MARIADB_PASSWORD=m_nasir5555"
- "MARIADB_HOST=mariadb"
volumes:
- type: 'bind'
source: './volumes/app'
target: '/app'
depends_on:
- "mariadb"
- "nginx"
mariadb:
image: 'mariadb:latest'
container_name: 'mariadb'
restart: 'always'
expose:
- '3306'
# ports:
# - "8000:8000"
environment:
- "MARIADB_DATABASE=django_app"
- "MARIADB_USER=peshgaman"
- "MARIADB_PASSWORD=m_nasir5555"
- "MARIADB_ROOT_PASSWORD=m_nasir5555"
volumes:
- type: 'bind'
source: './volumes/dbdata'
target: '/var/lib/mysql'
nginx:
build: './nginx'
container_name: 'nginx'
restart: 'always'
ports:
- "80:80"
volumes:
- type: 'bind'
source: './volumes/media'
target: '/app/media'
- type: 'bind'
source: './volumes/static'
target: '/app/static'
app Dockerfile
FROM python:alpine
ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1
ENV DJANGO_SUPERUSER_PASSWORD=admin
RUN mkdir /app
COPY requirements.txt /app
WORKDIR /app
RUN apk update
RUN apk add --no-cache gcc python3-dev musl-dev mariadb-dev
RUN pip install --upgrade pip
RUN pip install -r requirements.txt mysqlclient
RUN apk del gcc python3-dev musl-dev
CMD python3 manage.py makemigrations --noinput && \
while ! python3 manage.py migrate --noinput ; do sleep 1 ; done && \
python3 manage.py collectstatic --noinput && \
python3 manage.py createsuperuser --user admin --email admin#localhost --noinput; \
gunicorn -b 0.0.0.0:8000 config.wsgi
nginx Dockerfile
FROM nginx:alpine
RUN rm /etc/nginx/conf.d/default.conf
ADD default.conf /etc/nginx/conf.d/default.conf
nginx defaul.conf
upstream app {
server app:8000;
}
server {
listen 80;
location / {
proxy_pass http://app;
}
location /media {
alias /app/media;
}
location /static {
alias /app/static;
}
}