Learn about images & containers

Docker Engine provides the core Docker technology that enables images and containers. As the last step in your installation, you ran the docker run hello-world command. The command you ran had three parts.

An image is a filesystem and parameters to use at runtime. It doesn’t have state and never changes. A container is a running instance of an image. When you ran the command, Docker Engine:

• checked to see if you had the hello-world software image
• downloaded the image from the Docker Hub (more about the hub later)
• loaded the image into the container and “ran” it

Depending on how it was built, an image might run a simple, single command and then exit. This is what Hello-World did.

A Docker image, though, is capable of much more. An image can start software as complex as a database, wait for you (or someone else) to add data, store the data for later use, and then wait for the next person.

Who built the hello-world software image though? In this case, Docker did but anyone can. Docker Engine lets people (or companies) create and share software through Docker images. Using Docker Engine, you don’t have to worry about whether your computer can run the software in a Docker image — a Docker containercan always run it.