Docker has three core nouns. Learn these before memorising commands: image, container, and registry.

Image

An image is a read-only package used to create containers. Think of it as the saved blueprint for your application.

It contains:

• Filesystem layers.
• Runtime dependencies.
• Application code.
• Metadata.
• Default command.

Images are built from Dockerfiles.

Container

A container is a running instance of an image—your app actually running.

From one image, you can start many containers.

Each container has its own writable layer, process space, network identity, and lifecycle.

Registry

A registry stores Docker images so another machine can download and run them. Think of it as a package store for images.

Examples:

• Docker Hub.
• GitHub Container Registry.
• Amazon Elastic Container Registry.

Teams push built images to a registry, then deployment systems pull those images.

Tags

Tags identify image versions.

docker build -t my-api:1.0 .
docker tag my-api:1.0 registry.example.com/my-api:1.0
docker push registry.example.com/my-api:1.0

Avoid relying only on latest in production because it is mutable and can make deployments hard to reproduce.

Image layers

Docker images are built in layers. If a layer does not change, Docker can reuse it from cache.

This is why Dockerfile instruction order matters. Copy dependency manifests before application source when possible.

Quick revision

• Image is the package.
• Container is the running instance.
• Registry stores and distributes images.
• Tags label image versions.
• Avoid mutable production deployments based only on latest.