Nginx gives 502 Bad Gateway when proxying to nodejs app running on different docker container

I am using docker compose to create 2 containers one for application running on Nginx and another for backend application on Nodejs(running on another port 4000) with routes of patterns “/org-metadata/, /proxy-api/, /node-api/**”.

I am trying to proxy the nginx to nodejs running on 4000, but it does not reach the backend. and the nginx gives the 502 Bad Gateway response.

P.S: my node app gives 503 service unavailable when I run it from browser.

Can some one pls point where am I missing anything?

Here is my nginx.conf file:

worker_processes auto;

events {
    worker_connections 8000;
    multi_accept on;
}

http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    access_log /dev/stdout;
    error_log /dev/stderr;

    upstream backend {
        server 127.0.0.1:4000;
    }

    server {
        listen 80;

        root /var/www;
        index index.html index.htm;

        location / {
            try_files $uri $uri/ /index.html;
        }

        location ~ (proxy-api|node-api|\/api\/) {
            proxy_pass http://backend;
            proxy_redirect off;
            add_header X-Frame-Options "SAMEORIGIN" always;
            add_header X-XSS-Protection "1; mode=block" always;
            add_header X-Content-Type-Options "nosniff" always;
            add_header Referrer-Policy "no-referrer-when-downgrade" always;
            add_header Content-Security-Policy "default-src * data: 'unsafe-eval' 'unsafe-inline'" always;
        }
    }
}

docker-compose.yml

version: "3"

services:
    nodejs:
        build:
            context: "./server"
        ports:
            - "4000:4000"
        image: nodejs
        container_name: nodejs
        restart: unless-stopped
    webserver:
        build: "."
        ports:
            - "80:80"
        image: webserver
        container_name: webserver
        restart: unless-stopped
        depends_on:
            - nodejs

Dockerfile for nodejs:

FROM node:11-alpine
WORKDIR /app
COPY . .
RUN npm ci
EXPOSE 4000
CMD [ "npm", "run", "start" ]

Dockerfile for nginx:

FROM nginx:1.15.2-alpine
COPY ./build /var/www
COPY nginx.conf /etc/nginx/nginx.conf
EXPOSE 80
ENTRYPOINT ["nginx","-g","daemon off;"]