= Container image
A <container image> packages a filesystem description and execution configuration used to create software <software containers>[containers]. It contains such things as application <files>, <software libraries>[libraries] and a default command. Image layers and content identifiers permit sharing unchanged content; a running <software container>[container] adds execution state and usually a separate writable layer.<What is an image? in the Docker documentation>[Explanation, immutability and layers]{ref}
For example, two <software containers>[containers] can start from the same image while receiving different <input> <files> and environment settings. Their results need not match merely because their image matches. A <container image digest> identifies particular image content, while a movable tag can later refer to a different image. Most <software containers>[containers] share a host kernel, so an image is not necessarily a complete guest operating-system installation. Reproducing a calculation also requires its <inputs>, configuration and compatible host facilities.
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