A docker image contains a bundle of files and dependencies required to run an application.
An example image is a minimal Apache httpd installation.
Images can be extended from existing parent images, with files being add or modified.
For example, a website image might use the Apache httpd image as a base image, and add static HTML. Now the image contains enough files to serve the website.
An image can only extend one parent image - there is no multiple inheritance. Though that parent image can extend from another parent image and so on.
Common base or parent images are usually a Linux distro with a minimal feature set such as Alpine linux, or a barebones installation of Debian or Ubuntu. Often a piece of software will be published as images with a choice of multiple parent images. Apache httpd is published with a choice of Alpine or Debian as the parent image.
Building an image
Images should be built using a Dockerfile.
Alternatively, images can be built from an existing container that has been modified or upgraded by using
docker commit. Before doing this, consider whether volume management is more appropriate.