Skip to content

Day-02.md

Latest commit

 

History

History
86 lines (69 loc) · 2.75 KB

Day-02.md

File metadata and controls

86 lines (69 loc) · 2.75 KB

Day 2: Dockerfiles - Building Custom Images

Concepts:

  • Dockerfile: A text file containing instructions for building a Docker image. Each instruction creates a layer in the image.
  • Build Context: The directory containing the Dockerfile and any files it needs to copy into the image.
  • Layers & Caching: Docker images are composed of layers, where each instruction in a Dockerfile adds a new layer. This allows for efficient caching during builds.
  • Common Dockerfile Instructions: FROM, WORKDIR, COPY, ADD, RUN, CMD, ENTRYPOINT, EXPOSE.

Examples:

  1. Create a simple Node.js application: Create a directory named my-node-app. Inside, create app.js:
// app.js
const http = require('http');
const port = 3000;

const server = http.createServer((req, res) => {
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello from Docker!\n');
});

server.listen(port, () => {
    console.log(`Server running at http://localhost:${port}/`);
});

And package.json:

{
    "name": "my-node-app",
    "version": "1.0.0",
    "description": "A simple Node.js app",
    "main": "app.js",
    "scripts": {
    "start": "node app.js"
    },
    "author": "",
    "license": "ISC"
}
  1. Create a Dockerfile in the same directory:
# Use an official Node.js runtime as a parent image
FROM node:18-alpine

# Set the working directory in the container
WORKDIR /usr/src/app

# Copy package.json and package-lock.json to install dependencies
# This is done first to leverage Docker's build cache for dependencies
COPY package*.json ./

# Install app dependencies
RUN npm install

# Copy the rest of the application code
COPY . .

# Expose the port the app runs on (documentation for humans, not functional)
EXPOSE 3000

# Define the command to run the application when the container starts
CMD [ "npm", "start" ]
  1. Build the Docker image:
sudo docker build -t my-node-app:1.0 .

(The -t flag tags the image with a name and optional version, and . specifies the build context.)

  1. Run a container from your custom image:
docker run -p 8000:3000 --name my_running_app my-node-app:1.0

(Access http://localhost:8000 in your browser. Use Ctrl+C to stop the container.)

Challenge 2: Containerize a Python Flask App

  • Create a simple Python Flask application that returns "Hello from Flask in Docker!" on port 5000.
  • Write a Dockerfile to containerize this Flask application.
  • Build the image and run a container, ensuring you can access the Flask app from your host browser.
  • Experiment with CMD vs ENTRYPOINT. What happens if you try to docker run <your_image_name> echo "Hello" when using CMD vs ENTRYPOINT?