Docker image 

 

 

A Docker image is a file, comprised of multiple layers, that is used to execute code in a Docker container. An image is essentially built from the instructions for a complete and executable version of an application, which relies on the host OS kernel. When the Docker user runs an image, it can become one or multiple instances of that container.

Docker is an open source OS-level virtualization software platform primarily designed for Linux, Windows and MacOS. Sometimes moving software from one host machine to another will lead to issues because of any dependencies the software may have had. A container that moves from one Docker environment to another with the same OS will work without any changes, since the image includes all of the dependencies needed to execute the code. Docker will use resource isolation features in the OS kernel, such as cgroups in Linux, to run multiple independent containers on the same OS.

It is also important to note that containers differ from virtual machines (VMs), which encapsulate an entire OS with the executable code atop an abstraction layer from the physical hardware resources.
A Docker host can be either a virtual or a physical machine running the host operating system. The building and running of containers on the host is handled by a Docker daemon.

Confusion sometimes occurs around Docker images and containers dealing with disk space. Size and virtual size do have different meanings. Size refers to the disk space used by the writable layer of each container. On the other hand, the disk space needed for the container and the writeable layer is called the "virtual size." This is because the read-only layers of an image can be shared between any container started from the same image.