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* Added User Guide section outlines. * Added User Guide to menu. * Moved HTTPS example to articles. * Replaced Hello World example with User Guide. * Moved use cases out of examples. * Updated Introduction to add User Guide. * Redirected migrated /use and /articles links. * Added Docker.io section * Added Dockerized section * Added Using Docker section * Added Docker Images section * Added Docker Links section * Added Docker Volumes section Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
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page_title: Images page_description: Definition of an image page_keywords: containers, lxc, concepts, explanation, image, container
Image
Introduction
In Docker terminology, a read-only Layer is called an image. An image never changes.
Since Docker uses a Union File System, the processes think the whole file system is mounted read-write. But all the changes go to the top-most writeable layer, and underneath, the original file in the read-only image is unchanged. Since images don't change, images do not have state.
Parent Image
Each image may depend on one more image which forms the layer beneath it. We sometimes say that the lower image is the parent of the upper image.
Base Image
An image that has no parent is a base image.
Image IDs
All images are identified by a 64 hexadecimal digit string (internally a 256bit value). To simplify their use, a short ID of the first 12 characters can be used on the command line. There is a small possibility of short id collisions, so the docker server will always return the long ID.