How to flatten a Docker image?

Introduction

Flattening a Docker image is the process of creating a new, single-layer image from an existing multi-layer image. Flattening an image can be useful in a variety of scenarios, such as reducing the size of the image, improving the performance of the image, or simplifying the image for easier distribution.

A flattened Docker image is an image that consists of a single layer, containing all the files and metadata required for the image to function. This is in contrast to a multi-layer image, which consists of multiple layers stacked on top of each other, each containing a subset of the files and metadata required for the image.

Benefits of flattening a Docker image

There are several benefits to flattening a Docker image −

Reduced size − Flattening an image can significantly reduce its size, as it removes the overhead of multiple layers. This can be especially useful for large images or images that are intended to be distributed over a network.
Improved performance − Flattening an image can improve the performance of the image, as it reduces the number of layers that must be processed during runtime. This can be especially useful for images that are used frequently or in high-performance environments.
Simplified distribution − Flattening an image can simplify the process of distributing the image, as it removes the need to manage multiple layers and their dependencies. This can be especially useful for images that are intended to be shared or reused by multiple users.

Tools and methods for flattening a Docker image

There are several tools and methods available for flattening a Docker image. Some of the most popular tools and methods include −

docker export − The docker export command creates a tar archive of the contents of a Docker image, without the metadata or history of the image. This can be used to create a flattened copy of the image.
docker save − The docker save command creates a tar archive of the entire image, including its metadata and history. This can be used to create a flattened copy of the image, but the resulting image will include the metadata and history of the original image.
docker-squash − docker-squash is a third-party tool that can be used to flatten a Docker image. It works by creating a new image that consists of a single layer containing the files and metadata from the original image. docker-squash offers several advanced features, such as the ability to specify which layers to include or exclude from the flattened image, and the ability to modify the metadata of the flattened image.

Example1: Using docker export

To flatten a Docker image using docker export, follow these steps −

Identify the ID of the Docker image you want to flatten. You can use the docker images command to list the available images and their IDs.

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
myimage             latest              abcdef123456        1 hour ago          1GB

Use the docker export command to create a tar archive of the image. Replace <image_id> with the ID of the image you want to flatten.

$ docker export <image_id> > myimage.tar

Use the docker import command to create a new Docker image from the tar archive. Replace <image_name> with the desired name for the new image.

$ docker import myimage.tar <image_name>:latest

This will create a new Docker image with the same contents as the original image, but without the metadata or history of the original image. The resulting image will be a flattened copy of the original image.

Example 2: Using docker-squash

To flatten a Docker image using docker-squash, follow these steps −

Install docker-squash by following the instructions on the project's GitHub page (https://github.com/jwilder/docker-squash).
Identify the ID of the Docker image you want to flatten. You can use the docker images command to list the available images and their IDs.

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
myimage             latest              abcdef123456        1 hour ago          1GB

Use the docker-squash command to create a new, flattened image from the original image. Replace <image_id> with the ID of the image you want to flatten, and <image_name> with the desired name for the new image.

$ docker-squash -i <image_id> -o <image_name>

This will create a new Docker image with the same contents as the original image, but with a single layer containing all the files and metadata from the original image. The resulting image will be a flattened copy of the original image.

Best practices for flattening a Docker image

Here are some tips for choosing the right tool or method for flattening a Docker image −

If you need to preserve the metadata or history of the original image, consider using docker save instead of docker export.
If you need advanced features such as layer selection or metadata modification, consider using docker-squash instead of docker export.
If you are unsure which tool or method to use, consider testing multiple options and comparing the results to find the best fit for your needs.

Here are some strategies for optimizing image size and performance when flattening a Docker image −

Consider using a multi-stage build to create a smaller base image, and then adding only the necessary files and dependencies in the final image. This can help to reduce the size of the flattened image.
Consider using a lightweight base image, such as Alpine Linux or scratch, as the foundation for your image. This can help to reduce the size and improve the performance of the flattened image.
Consider using image optimization tools, such as upx, to further reduce the size of the flattened image.

Conclusion

In this article, we looked at how to flatten a Docker image. We saw that flattening an image can be useful in a variety of scenarios, such as reducing the size of the image, improving the performance of the image, or simplifying the image for easier distribution. We also covered several tools and methods for flattening an image and provided examples of using docker export and docker-squash to flatten an image. Finally, we discussed some best practices for choosing the right tool or method for flattening an image, and for optimizing the size and performance of the resulting image.