Deploy node application whit docker and nginx

I am looking into a lot of content on the internet how to solve the following situation; many blogs are incomplete and don’t get right to the point. Let’s see who can help me.

I have a web application written in react, so we are in the context of the node.

So once tested, locally or on a server we write the Dockerfile for production named as Dockerfile.prod

Many blogs put both the app source code and the nginx processes in this file. You will then have the following file:

FROM node:16-alpine as builder

WORKDIR /app

COPY package*.json ./

RUN npm install

COPY . .

RUN npm run build

FROM nginx:1.22-alpine as nginx

COPY --from=builder /app/build /usr/share/nginx/html 

COPY /nginx.conf /etc/nginx/conf.d/default.conf

I believe it can also be done differently,ie create two distinct docker images and start them in the docker-compose file. I found some examples but not very reliable, although I think it is the right way, as it gives you more control for each process. So if by any chance someone wants to write the steps that would be great.

Returning to our problem, let’s write the docker-compose for an image that will be provided by the docker file written above.

version: '3.8'
services:
  website:
    container_name: my-website
    build:
      context: .
      dockerfile: Dockerfile.prod
    ports:
      - "80:80"
    restart: unless-stopped

Once this is done, we create the docker image, which will then be pushed into the docker-hub(private).I leave out the commands. We get to the problem: we create a project on DigitalOcean, we create a droplet, we make all the most suitable configurations, we also buy a domain mydomain.com on some supplier for example, we set the dns for the droplet. We enter with the ssh key, we pull our image from the docker hub and finally we have to start it. Now my question is if a container opens on a door with the flag -p 8080:80 for example; to find my webapp I have to write mydomain.com:8080 instead of just writing mydomain.com. How is this problem solved?

PS: I didn’t put many commands or all processes to do for security.

I inform you that I have also read this article, but I do not like the initial phase because it does not use the build command.