Images and Containers

Introduction

The two most fundamental concepts in Docker are Images and Containers. Understanding the difference between them is critical for working with Docker effectively.

What is a Docker Image?

An Image is a read-only template with instructions for creating a Docker container. It contains the application code, libraries, dependencies, tools, and the operating system files needed to run an application.

Think of an image like a class in Object-Oriented Programming, or a blueprint for a house.

You build images using a Dockerfile.

# A simple Dockerfile
FROM node:18-alpine
WORKDIR /app
COPY . .
RUN npm install
CMD ["npm", "start"]

You can build this image using the command:

docker build -t my-node-app .

What is a Docker Container?

A Container is a runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI.

Think of a container like an instantiated object of a class, or the actual physical house built from the blueprint. You can run multiple containers from a single image simultaneously.

To start a container from the image we just built:

# Run the container and map port 3000 on the host to 3000 in the container
docker run -p 3000:3000 -d my-node-app

Useful Commands

• docker images: Lists all images on your local machine.
• docker ps: Lists all currently running containers.
• docker ps -a: Lists all containers (running and stopped).
• docker stop <container_id>: Gracefully stops a running container.

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