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In this article, we will understand the difference between containers and images by showing the components that make up containers and images and the features that make them different.

Understanding Docker layers

To create a custom image, we usually use a Dockerfile, which defines the instructions for creating a custom image using a base image. The instructions that can be used to build an image in a Dockerfile are as follows.

FROM node:16.17.0-alpine
WORKDIR /app
ADD package*.json ./
RUN npm install
ADD . .
CMD node index.js

Docker images are created using layers stacked on top of each other. When the instructions in the Dockerfile act to add or remove files, a new layer is added to the stack.

请注意, not all instructions defined in a Dockerfile create a new layer. For example, the CMD instruction adds metadata about the command to be run inside the container to the image.

Also note that all layers created using a Dockerfile are read-only except for the last layer.

When we create a container, a new thin layer is created. The difference between this layer and the previous one is that we can read and write files from this layer.

After the container is running, operations such as writing new files, modifying existing files, and deleting files are all performed in the thin read-write layer. The following figure shows the stack containing the read-only layer of the image and the read-write layer of the container.

The difference between Docker containers and images

In the previous section, we have seen that the top layer of the stack is a thin read-write layer used by containers, and the other layers are read-only layers used to create custom images.

This is the main difference between Docker containers and images. Any changes that add or remove files in a container are made to a thin read-write layer for that specific container.

Note that deleting a container deletes its associated layers. However, the base image is not deleted.

Each container created has its own read-write layer, allowing them to share the base image but maintain their data state. The following diagram shows how multiple containers can share a base image.

Summarize

In this article, we have learned how to differentiate between docker containers and docker images by using layers. We have learned that images are created using a file called Dockerfile and each instruction in the file forms a read-only layer stacked on top of another layer.

Finally, we learned that the top layer is a read-write layer that is used by containers to add or modify file changes.