Container Image and Container

A container image packages filesystem contents and execution metadata. A container is an instance created from an image, with runtime configuration and a writable layer. Several containers can start from the same image without sharing that writable layer.

A Linux container runs with a Linux kernel supplied by the host or a virtual machine. The image does not include every external dependency: CPU architecture, mounts, credentials, network services, and input versions still affect the job.

Removing a container removes its writable-layer data. Persist required output through an explicitly managed mount or external destination. Keeping an image is not a backup of the data produced by its containers.

The lifecycle is build or pull an image, create a container from it, start its process, then stop or remove the container. A stopped container can still exist with its writable layer; removal is the step that discards that layer. Changing a running container does not update the original image or other containers created from it. Rebuild an image to distribute a reviewed application change.

See Containerizing Data Jobs with Docker for a worked example.

Further reading

• Docker: What is a container?