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Understanding Docker Images

In Docker, an image is the blueprint for a container. It contains everything needed to run an application: code, runtime, system tools, libraries, and settings. Think of it like a recipe that never changes – you use it to cook many dishes (containers).

What Exactly Is a Docker Image?

An image is a read‑only template with multiple layers. Each layer represents a set of filesystem changes – for example, installing a package or adding a file. Layers are stacked, and Docker reuses them across images to save space and speed up builds.

A Docker image is a lightweight, standalone, executable package that includes everything needed to run a piece of software.

Image Layers

If you run docker history ubuntu, you'll see the layers that make up the Ubuntu image. Each layer is a read‑only filesystem. When you run a container, Docker adds a thin writable layer on top, but the original image remains unchanged.

Image Names and Tags

Images are identified by repository and tag. For example:
  • ubuntu:22.04 – Ubuntu 22.04 version
  • nginx:latest – latest Nginx
  • myapp:v1.0 – your custom app
If you omit the tag, Docker assumes latest.

Listing and Inspecting Images

List images you have locally:
docker images
To see detailed information about an image:
docker inspect ubuntu

Pulling an Image

When you run a container, Docker automatically pulls the image if it's not present. You can also manually pull:
docker pull python:3.11

Removing an Image

If you no longer need an image, delete it with:
docker rmi image_name
You cannot remove an image that is used by any running or stopped container. Remove the container first.


Two Minute Drill
  • Images are read‑only templates for containers.
  • Images consist of layers, which are reused for efficiency.
  • Tags identify versions, e.g., ubuntu:22.04.
  • List images with docker images, remove with docker rmi.

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