Flattening Docker images – Creating and Managing Container Images

Docker inherently uses a layered filesystem, and we have already discussed why it is necessary and how it is beneficial in depth. However, in some particular use cases, Docker practitioners have observed that a Docker image with fewer layers performs better. You can reduce layers in an image by flattening it. However, it is still not a best practice, and you need to do this only if you see a performance improvement, as this would result in a filesystem overhead.

To flatten a Docker image, follow these steps:

1. Run a Docker container with the usual image.
2. Do a docker export of the running container to a .tar file.
3. Do a docker import of the .tar file into another image.

Let’s use the nginx-hello-world image to flatten it and export it to another image; that is, <your_dockerhub_user>/nginx-hello-world:flat.

Before we move on, let’s get the history of the latest image:

Now, let’s run a Docker image with the latest image:

$ docker run -d –name nginx <your_dockerhub_user>/nginx-hello-world:latest e2d0c4b884556a353817aada13f0c91ecfeb01f5940e91746f168b…

Next, let’s take an export out of the running container:

$ docker export nginx > nginx-hello-world-flat.tar

Import nginx-hello-world-flat.tar to a new image; that is, <your_dockerhub_user>/ nginx-hello-world:flat:

$ cat nginx-hello-world-flat.tar |   \

docker import – <your_dockerhub_user>/nginx-hello-world:flat sha256:57bf5a9ada46191ae1aa16bcf837a4a80e8a19d0bcb9fc…

Now, let’s list the images and see what we get:

Here, we can see that the flat image is present and that it occupies less space than the latest image. If we view its history, we should see just a single layer:

$ docker history <your_dockerhub_user>/nginx-hello-world:flat

IMAGE                CREATED                       CREATED BY  SIZE      COMMENT

57bf5a9ada46  About a minute ago                        177MB    Imported from –

It has flattened the image. But is it a best practice to flatten Docker images? Well, it depends. Let’s understand when and how to flatten Docker images and what you should consider:

• Are several applications using a similar base image? If that is the case, then flattening images will only increase the disk footprint, as you won’t be able to take advantage of a layered filesystem.
• Consider alternatives to flattening images using a small base image, such as Alpine.

• Multi-stage builds are helpful for most complied languages and can reduce your image’s size considerably.

• You can also slim down images by using as few layers as possible by combining multiple steps into a single RUN directive.

• Consider whether the benefits of flattening the image outweigh the disadvantages, whether you’ll get considerable performance improvements, and whether performance is critical for your application needs.

These considerations will help you understand your container image footprint and help you manage container images. Remember that although reducing the image’s size is ideal, flattening it should be a last resort.

So far, all the images we’ve used have been derived from a Linux distribution and always used a distro as their base image. You can also run a container without using a Linux distro as the base image to make it more secure. We’ll have a look at how in the next section.