Docker Intro for DUMMIES (like me)

I don’t know why, but Docker seems confusing for me, that’s why I started learning some more about it. As a senior frontend engineer, I don’t like being at the hand of the senior backend so this is what I learned so far:

Let’s say I want to create a simple Nodejs app. My project is located in “D:/Projects/docker-tutorial”

1. Create an app/index.js for the Nodejs app.

const express = require('express');
const app = express();

app.get('/', (req, res) => {
    res.send('Hello World from node');
});

app.listen(8080, () => {
    console.log('Server is running on port 8080 ');
});

2. Next create a package.json

{
  "name": "docker-tuts",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.21.1"
  }
}

What I want to do is take this small app, copy it somewhere, install the dependencies and run it. For this, I will use Docker.

3. Create a file called dockerfile

FROM node:22-alpine

WORKDIR /home/node/app

COPY package*.json ./

RUN npm install

COPY app/ .

CMD ["node", "index.js"]

This is the “recipe” for your containers. This will create an image.

The image is the recipe. The image represents the instructions to build the containers.

The containers are the dishes. The containers contain the logic and node_modules.

First let’s explain what is happening:

Step 1: Specify the Base Image

FROM node:22-alpine

We start from the official Node.js image based on Alpine Linux for a lightweight container.

Step 2: Set the Working Directory

WORKDIR /home/node/app

This sets the working directory inside the container where all subsequent commands will be executed and where your logic and node_modules will stay.

Step 3: Copy the package.json

COPY package*.json ./

This copies package.json and package-lock.json (if it exists) into the working directory.

Step 4: Install Dependencies

RUN npm install

Installs the dependencies specified in package.json inside the working directory.

Step 5: Copy Application Code

COPY app/ .

Copies your application code app/index.js into the container /home/node/app/.

Step 6: Specify the Command to Run the App

CMD ["node", "index.js"]

Defines the command to start your application when the container starts.

The behavior of the COPY command depends on whether you include a trailing slash:

• With trailing slash COPY app/ ./destination/: Copies the contents of the app directory into a new directory called destination, inside the container resulting in /home/node/app/destination/index.js.

•     # Result in container:
      # /home/node/app/destination/
      # └── index.js
  

• Without trailing slash COPY app ./destination/: Copies the entire app directory into /home/node/app/destination/, resulting in /home/node/app/destination/app/index.js.

•     # Result in container:
      # /home/node/app/destination/
      # └── app/
      #     └── index.js
  

• Using just . or ./ for the destination COPY app/ .: Copies the contents of the app directory into the current directory inside the container (/home/node/app/), so index.js ends up directly inside /home/node/app/.

•     # Result in container:
      # /home/node/app/
      # └── index.js
  

In the console you create the image

In “D:/Projects/docker-tutorial” you can run

docker build . -t myNodeImg

• This will build the image inside docker and this image will contain the instructions

• -t myNodeImg - will give the name of the image

• notice the . - this is the directory where the dockerfile exists

Then you can create a container (or multiple) with

docker run --name myNodeContainer1 -p 80:8080 -d myNodeImg

• This will create a named container named myNodeContainer1

• it will link the port (-p) from your computer to the port inside the container, where -p <host_port>:<container_port>. In this case, -p 80:8080 maps port 80 on your computer (host) to port 8080 inside the container.

• it will not block your terminal, by being detached from it (-d)

• it will know to create the container from the image myNodeImg

Now when you go to the browser on http://localhost:80 you should see the message “Hello World from node”.

That’s it for now but stay tuned for more.