Docker User Guide: Getting Started with Containers

By enabling applications to run in isolated containers, Docker simplifies the development, deployment, and scaling of applications. In this guide, we'll explore the basics of Docker

1. What is Docker?

Docker is a platform that allows developers to package applications and their dependencies into containers, ensuring consistency across development, testing, and production environments. Unlike traditional virtual machines (VMs), containers share the host system’s kernel, making them lightweight and faster to start.

2. Installing Docker

Getting Docker up and running is easy:

Linux: https://www.docker.com/

docker --version        

3. Basic Docker Commands

• docker run: Run a container from an image.
• docker images: List all downloaded images.
• docker ps: List running containers.
• docker pull: Download an image from Docker Hub.
• docker stop and docker rm: Stop and remove containers.

4. Creating Your First Docker Container

Let’s spin up a simple web server using nginx:

docker run -d -p 8080:80 nginx        

-d: Runs the container in detached mode.

5. Building Docker Images

One of Docker's key strengths is building custom images using a Dockerfile. Here’s an example for a Python Flask app:

FROM python:3.9
WORKDIR /app
COPY . /app
RUN pip install -r requirements.txt
CMD ["python", "app.py"]
        

Build the image:

docker build -t flask-app .        

6. Docker Compose: Managing Multi-Container Apps

With Docker Compose, you can define and run multi-container Docker applications. Here’s a simple docker-compose.yml for a web app with a PostgreSQL database:

version: '3'
services:
  web:
    image: flask-app
    ports:
      - "5000:5000"
  db:
    image: postgres:latest
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
        

Start the application:

docker-compose up