Docker Image: why page not accessible?

Question

I am trying to create a Docker image for a little API I built with Fastify and Node. I came up with the following Dockerfile:

FROM node:current-alpine3.19

WORKDIR /usr/src/app

RUN chown -R node:node /usr/src/app

COPY src/package.json ./
COPY src/package-lock.json ./

RUN npm install

USER node

COPY --chown=node:node ./src .

CMD [ "npm", "start" ]

The build worked and I started the container with "docker run -p 3000:3000 [my-project]".

The file structure seems to be correct and I get the expected response when trying "wget 127.0.0.1:3000" inside the container.

But I can't open the page from my PC with localhost:3000 or 127.0.0.1:3000. Firefox: "The connection to the server was reset while the page was loading." Curl: "curl: (56) Recv failure: Connection reset by peer"

Fastify runs under port 3000, port 3000 is also used in the "docker run" command and I open the page with port 3000.

What am I doing wrong?

Answer 1

You need to accept connections from 0.0.0.0 because 127.0.0.1 references the container and not the host.

So, for example, supposing that your project looks like this (very simplified!):

├── Dockerfile
└── src
    ├── index.js
    ├── package.json
    └── package-lock.json

then your index.js might be:

const fastify = require('fastify')({
    logger: true
  });
  
  fastify.get('/', async (request, reply) => {
    return { hello: 'world' }
  });
  
  const start = async () => {
    try {
      await fastify.listen({ port: 3000, host: '0.0.0.0' });
      console.log(`Server is running at ${fastify.server.address().port}`);
    } catch (err) {
      fastify.log.error(err);
      process.exit(1);
    }
  }
  start();

The important bit is specifying host: '0.0.0.0' to fastify.listen(). You also need to share this port with the host when you do docker run by giving -p 3000:3000.