What is an image file in docker

Docker is a well-known open source containerization platform that is designed to allow applications to run stably in different environments. In Docker, the image file is a very important part. It is used to store the running environment of the application, including the operating system, software applications and configurations.

In Docker, an image file is composed of multiple UnionFS layers and is managed through the hierarchical structure of the image file. Each hierarchy is read-only and they are called union file system layers. This means that the image file can be shared by different containers without changing the original hierarchy. This is because when the container starts, it creates a partially writable file system by reading the hierarchical structure of the image file to store data inside the container.

The hierarchical structure of the image file is used by multiple Docker containers. Each container has its own writable layer, called the container layer (container layer) or usage layer (usage layer). The container layer is a new layer created on top of the image file hierarchy and is used to store modifications made to the container during runtime, such as installing new software packages, modifying configuration files, etc. Modifications made in the container are stored in this unique container layer and will not have any impact on the image file and other containers.

In addition, the container can also use Union mount to combine the hierarchical structure of the image file and the hierarchical structure of the container to jointly present the container's file system. This method allows the container to only use the hierarchical structure that has been prepared before, some of which are automatically created using the hierarchical structure of the image file when the container is running. Containers do not need to be updated or maintained when using these hierarchies.

In general, Docker image files are used to store the running environment of applications. They are composed of multiple joint file system layers and are managed through a hierarchical structure. Containers are created based on image files. They share the image files with the container through read-only image files, writable container layers and joint mounting. Based on these mechanisms, Docker can support applications to run stably and be deployed quickly in different environments.

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