How to write a .gitlab-ci.yml file to connect Nginx to Node.js?

I have two servers as follows:

GitLab: GitLab Server
Runner: Docker and GitLab-Runner

I created a Node.js file on the GitLab server and I want to run this file through Nginx and Node.js containers. I know that I should have two Dockerfile and .gitlab-ci.yml files on the GitLab server.
The Dockerfile is as follows:

FROM node:latest as build-stage
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY package*.json /usr/src/app/package.json
RUN npm install
RUN npm update
COPY . /usr/src/app

#production stage
FROM nginx:latest as production-stage
COPY --from=build-stage /usr/src/app /usr/share/nginx/html
COPY ./cfiles/default.conf /etc/nginx/conf.d/default.conf
CMD nginx -g 'daemon off;'

The default.conf is as follows:

upstream nodejs {
    server nodejs:3000;

server {
   listen 80;
   server_name default_server;
   error_log  /var/log/nginx/error.system-default.log;
   access_log /var/log/nginx/access.system-default.log;
   charset utf-8;

    root /usr/share/nginx/html;
    index index.html index.php index.js;

location ~ \.js$ {
    proxy_pass http://nodejs;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;

    location / {
     autoindex on;
     try_files $uri $uri/ $uri.html =404;

But I don’t know what to do about the .gitlab-ci.yml file. I searched the internet for examples, but none of them related to what I want to do.
Please advice me.

Thank you.

The “runner” will only “run” the build process. You need separate server(s) to “run” the resulting containers from the built Docker images.