What is a container image?

What is a container image?

A container image is a portable package that contains software. It's this image that, when run, becomes our container. The container is the in-memory instance of an image.

A container image is immutable. Once you've built an image, the image can't be changed. The only way to change an image is to create a new image. This feature is our guarantee that the image we use in production is the same image used in development and QA.

What is the host OS?

The host OS is the OS on which the Docker engine runs. Docker containers running on Linux share the host OS kernel and don't require a container OS as long as the binary can access the OS kernel directly.

However, Windows containers need a container OS. The container depends on the OS kernel to manage services such as the file system, network management, process scheduling, and memory management.

What is the container OS?

The container OS is the OS that is part of the packaged image. We have the flexibility to include different versions of Linux or Windows OSs in a container. This flexibility allows us to access specific OS features or install additional software our applications may use.

The container OS is isolated from the host OS and is the environment in which we deploy and run our application. Combined with the image's immutability, this isolation means the environment for our application running in development is the same as in production.

In our example, we're using Ubuntu Linux as the container OS and this OS doesn't change from development or production. The image we use is always the same.