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Difference Between a Docker Image and Docker Layers

In this article, we clarify the distinction between Docker images and Docker layers—two concepts that can often be confusing, especially during DevOps interviews. Docker images are built from multiple layers. However, Docker layers themselves are not standalone components; they are the building blocks that form a complete image. Each command in a Dockerfile—such as setting an environment variable or specifying an entry point—creates a new layer. These layers act like miniature images that include only the changes made relative to the previous layer.

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Example Dockerfile

Consider the following Dockerfile: 
FROM rails:onbuild
ENV RAILS_ENV dev
ENTRYPOINT ["bundle", "exec", "logica"]
  • Base Layer: The FROM rails:onbuild instruction generates the base layer.
  • Second Layer: The ENV RAILS_ENV dev instruction creates an additional layer by setting an environment variable.
  • Third Layer: The ENTRYPOINT instruction adds yet another layer that defines the container’s entry point.
When you build the Docker image, these layers are combined. You can inspect each layer individually with the docker history command, which displays the various layers contributing to the final image. This command helps you understand the construction of your image.
Understanding Docker layers can help you optimize your images by reducing size and improving security. Even though these layers operate behind the scenes, they are crucial for both image creation and container operation.
In summary, each Dockerfile instruction creates a new Docker layer, and these layers collectively form the complete Docker image. While you may not interact with these layers directly during container runtime, they are essential for efficient image building and maintenance. Now that you have a clear understanding of the difference between Docker images and Docker layers, you can confidently tackle this common interview question. Keep this concept in mind as you continue your DevOps interview preparation. Let’s proceed to the next section to explore additional topics in this series.

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