Docker Day 3: Master the Three Core Image Commands – pull, images, and rm

1. List Local Images

Use docker images or the shortcut docker image ls to display all images stored locally. The output includes columns for REPOSITORY (image name), TAG (version tag, default latest), IMAGE ID (unique identifier), CREATED, and SIZE.

2. Pull Images

Pulling without a tag defaults to the latest tag:

# Pull the latest Ubuntu image
docker pull ubuntu

# Pull a specific version
docker pull ubuntu:22.04
docker pull python:3.12-slim
docker pull mysql:8.0

# Pull from a private registry
docker pull myregistry.com/myimage:tag

Practical scenario: In CI/CD pipelines, running docker pull first caches layers and speeds up subsequent builds.

3. Remove Images

# Delete a single image
docker rmi nginx

# Force delete even if the image is in use
docker rmi -f ubuntu

# Remove all dangling images
docker image prune

# Delete every image (use with caution)
docker rmi $(docker images -q)

4. Tag Images

Because image IDs are hard to remember, assign a human‑readable tag:

# Official format: registry/username/repository:tag
docker tag hello-world myname/hello:1.0
# Verify the tag
docker images | grep hello

5. Image‑to‑Container Relationship

Image (read‑only) → Container (writable layer)
    ↓ COPY
Container changes affect only that container, not the underlying image.

Analogy: a class (template) versus an instance; modifying the instance does not alter the class definition.

6. Clean Up Unused Resources

If the development environment runs out of disk space, regularly prune:

# Remove stopped containers
docker container prune

# Remove build cache
docker builder prune

# Remove all unused images, containers, networks
docker system prune

# More thorough cleanup (including volumes)
docker system prune -a --volumes

Real‑world case: Running docker system prune before project delivery can free 10–50 GB of space.

7. Day 3 Summary

docker images

– list local images docker pull – download images from a registry docker rmi – delete images docker tag – assign a friendly tag docker system prune – clean up unused resources

Next up (Day 4): writing your first Dockerfile to package code into an image.