Make Nginx return custom error page when backend is down

I have Nginx working as reverse proxy and Django application behind it. Django app is connected to Nginx through UWSGI. Django app is deployed using Docker and gets down for a second or so during deployment. I would like to instruct Nginx to return custom 503 error page during that period.

Below is my Nginx config:

upstream server_django {
    server 0.0.0.0:8000;
}

server {
    listen 443 ssl http2;
    listen [::]:443 ssl http2;
    server_name server.com;

    location / {
        # required to serve static assets
        alias    /data/server.com/website_root/;
        try_files $uri @django;
    }

    location @django {
        uwsgi_pass  server_django;
        include     /etc/nginx/uwsgi_params;
    }
}

I have tried adding following config inside "server" block:

uwsgi_intercept_errors on;
error_page 502 503 504 =503 @django_is_down;
location @django_is_down {
    root /error_pages/server.com/;
    add_header Retry-After 10 always;
    index 503.html;
}

but it didn't help since Nginx stared serving default 404 page when backend is down.

How do I detect when Django is down in Nginx and serve custom error page when that happens?