Docker Exercise 1

Preparation

Copy the following files into a local directory on your system.

app.mjs

import express from 'express';

import connectToDatabase from './helpers.mjs'

const app = express();

app.get('/', (req, res) => {
  res.send('<h2>Hi there!</h2>');
});

await connectToDatabase();

app.listen(3000);

helpers.mjs

const connectToDatabase = () => {
  const dummyPromise = new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve();
    }, 1000);
  });

  return dummyPromise;
};

export default connectToDatabase;

package.json

{
  "name": "docker-start",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.17.1"
  }
}

Dockerfile

FROM node:14

WORKDIR /app

COPY . .

RUN npm install

EXPOSE 3000

CMD [ "node", "app.mjs" ]

Build the image

Open a CLI and navigate into the folder you copied the files to.

Run:

docker build .

The output will give you a container Id. Something like

=> => writing image sha256:307e51e493e71a

Run the container based on the image

Use the first few characters of the ID (copy enough so that it is unique) to run a container based on that image.

docker run -p 3000:3000 307e51e

You can then open the app on port 3000 of your local machine.

Try using different ports

Specify different ports for both the exposed container port aswell as the host port.

Make a change

Change the HTML output from the web server to something else. Run the container again and make sure you see the changes.

Optimize the Dockerfile

Make sure that npm install only executes when the package.json file changes.