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Docker Docker & Containers: From Zero to Production Docker Images Module 2 — Docker Images

What Is a Docker Image? Layers Explained Visually

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R Ram Pothuraju Reading…

What You'll Learn

Beginner

  • What a Docker image is (and how it differs from a container)
  • How layers work and why they save disk space
  • The difference between read-only image layers and the writable container layer
Writable Container Layer (ephemeral — deleted when container stops) Layer 4: COPY . /app (your code) 12 MB Layer 3: RUN npm install (dependencies) 145 MB Layer 2: RUN apt-get install curl 8 MB Layer 1: FROM ubuntu:22.04 (base image) (read-only, shared across all containers using it) 77 MB ↑ R/W Read-Only ↓ Key insight: Each instruction in your Dockerfile creates a new read-only layer. When you run a container, Docker adds a writable layer on top.
Docker image layers — read-only stack with a writable top layer per container

Image vs Container

ImageContainer
Read-only templateRunning instance of an image
Stored on disk, doesn't changeCreated when you docker run
Can be shared, pushed, pulledHas a writable layer on top
Like a class in OOPLike an object (instance)

How Layers Work

Each instruction in a Dockerfile creates one layer:

Dockerfiledockerfile
FROM ubuntu:22.04          # Layer 1: base image (77 MB)
RUN apt-get install curl   # Layer 2: adds curl (8 MB)
COPY app /app              # Layer 3: adds your code (12 MB)
CMD ["./app/run"]          # No new layer — metadata only

When you run docker run myapp, Docker stacks these read-only layers and adds a thin writable layer on top. Any changes the container makes go to this writable layer — the image is never modified.

Why Layers Matter

Three big benefits
  • Sharing: Two containers from the same image share all layers — no duplication.
  • Caching: If a layer hasn't changed, Docker reuses it during builds — faster builds.
  • Efficiency: Only changed layers are pushed/pulled — not the whole image.

Common Docker Image Commands

Terminalbash
docker images              # List local images
docker pull nginx          # Download an image
docker rmi nginx           # Remove an image
docker image prune         # Remove unused images
docker image inspect nginx # View image details
docker tag nginx myrepo/nginx:v1  # Tag an image

Image Naming Convention

Image name formatbash
[registry/]repository[:tag]

# Examples:
nginx                    # Docker Hub, official, latest
ubuntu:22.04             # Docker Hub, official, version 22.04
myuser/myapp:v1.0        # Docker Hub, user repo, version v1.0
ghcr.io/org/repo:latest  # GitHub Container Registry
Always pin versions
Don't use latest in production. It's a moving target — your image changes without warning. Pin to a specific version: nginx:1.25.3 instead of nginx:latest.

Practical Exercise

Run docker pull nginx:alpine
Run docker images — see the image and its size
Run docker history nginx:alpine — see each layer and its size
Run docker run -d --name web nginx:alpine
Run docker exec web touch /tmp/test — writes to the container's writable layer
Run docker stop web && docker rm web — the writable layer is deleted

Key Takeaways

  • An image is a read-only template. A container is a running instance with a writable layer on top.
  • Each Dockerfile instruction creates one layer.
  • Layers are shared across containers from the same image — saves disk and download time.
  • Layer caching speeds up builds — unchanged layers are reused.
  • Always pin image versions (nginx:1.25.3), avoid latest> in production.
Previously: Lesson 08 covered container lifecycle.
Today: You learned what images are and how layers work.
Next: Lesson 10 covers pulling images from Docker Hub.
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