Update compose networking for container-container communication

[TimeTravelerFromNow]

Description

This PR updates the documentation for
https://docs.docker.com/compose/how-tos/networking/
https://docs.docker.com/engine/network/drivers/bridge/

This would be helpful for developers familiar with local web development e.g. http://localhost:3000 for learning how to communicate between two containers in docker compose.

/compose/how-tos/networking

• Mention that http://localhost:3000 in local development needs to be changed to http://servicename:3000 when communicating between two services/containers

/engine/network/drivers/bridge/

• Created a link to the section about Container to container communication in the bridge networking docs. I was hesitant about this addition, but I thought it would be helpful in case someone reading the docs is not in the correct place

How I got here

For fun, I was learning react and got the Tic tac toe example working. Next, I wanted to see if I could deploy the app using Kamal 2. Problem was, Kamal 2 requires a healthcheck at the /up route returning 200. React doesn't do this out of the box, and you would need a NextJS routing to accomplish this, which SpBills managed to do: see the issue comment on the Kamal 2 repository. I didnt want to bloat the simple tic tac toe project, so I found how to proxy a request in a React app, using setupProxy middleware. I set up a go app on port 8081 for the healthcheck route, and had the react app running on port 8080 with this setupProxy code:

const { createProxyMiddleware } = require('http-proxy-middleware');

module.exports = function(app) {
  app.use(
    '/up',
    createProxyMiddleware({
      target: 'http://localhost:8081',
      changeOrigin: true,
    })
  );
};

I managed to achieve this locally (going to http://localhost:8080/up showed the static page from the go app with a 200. However, when using docker compose running two services, tictactoe and healthcheck the same test resulted in 504s.

networking documentation

Guessing that docker does something differently for inter-container communication, I began reading about networking in docker, I was scanning https://docs.docker.com/engine/network/drivers/bridge/ for something that would give me a hint how to achieve container to container communication.
I finally figured out that I needed to replace http://localhost:8081 with http://healthcheck:8081

This PR adds documentation about the familiar http://localhost:3000 URL needing to be http://servicename:3000 in docker compose for container to container communication.

Hopefully this familiarity can help out a developer to bridge the gap about URL naming so they can communicate between containers successfully.

Related issues or tickets

none

Reviews

• Technical review
• Editorial review
• Product review

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