Containerization with Docker: Images, Containers, and Docker Compose

Containers package an application and its dependencies into a lightweight, portable unit. Docker is the most widely used tool for building and running containers.

Table of Contents

• Docker Fundamentals
• Containers

Docker Fundamentals

Docker Fundamentals explain how to create images, run containers, and manage registries.

Why it is useful: Containers ensure consistent environments across development, testing, and production.

Example:

# Build an image from a Dockerfile
docker build -t myapp .

# Run a container from the image
docker run -d -p 8080:80 myapp

These commands illustrate building and running a containerized application. The -t flag tags the image with a name, -d runs it detached (in the background), and -p maps the host port 8080 to the container's port 80.

Common Mistakes

• Forgetting the . at the end of docker build, which tells Docker where the build context is.
• Leaving containers running and accumulating unused images; clean up with docker system prune.

Containers

Containers are isolated environments that encapsulate code, runtime, and dependencies, eliminating "works on my machine" issues.

Why it is useful: They provide portability and simplify deployment across diverse platforms.

Example:

services:
  web:
    image: myapp
    ports:
      - "8080:80"

This docker-compose file defines a service that runs the myapp container and maps port 8080 to the container's port 80. With docker compose up, you can start a multi-service stack from a single file.

Best Practices

• Use a .dockerignore file to keep build context small (skip node_modules, .git, etc.).
• Prefer small base images like alpine to reduce image size and attack surface.