How to Clear Docker Cache?

Docker is a powerful tool that allows developers to easily package, distribute, and run applications in containers. However, over time, Docker images can accumulate a lot of unnecessary data and files, which can significantly increase the image size and slow down the build process. In this blog post, we'll explore how to clean Docker cache to improve performance and optimize disk usage.

Understanding Docker Cache

Before we dive into the cleaning process, it's important to understand how Docker cache works. When you build a Docker image, Docker uses a build cache to speed up the build process. The build cache stores intermediate layers of the image, which are the layers that don't change frequently. This allows Docker to reuse these layers when building a new image, saving time and resources.

However, when you make changes to your code, Docker will need to rebuild the entire image, including the intermediate layers. If you have a large build cache, this can significantly slow down the build process and consume disk space.

Cleaning Docker Cache

To clean Docker cache, you have a few options. Here are some of the most common methods:

Use the Docker CLI

One way to clean Docker cache is to use the Docker CLI. You can use the following command to remove all the dangling images, which are images that are not associated with a container:

docker image prune -f

This command will remove all the images that are not associated with a container. You can also use the following command to remove all the unused images, which are images that are not being used by any container:

docker image prune -a -f

This command will remove all the unused images, including their intermediate layers. Be careful when using this command as it can remove images that you may still need.

Use a Dockerfile instruction

You can also clean Docker cache by adding a Dockerfile instruction to remove intermediate layers. Here's an example:

FROM base-image
RUN apt-get update && apt-get install -y \
    package1 \
    package2 \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/*

In this example, we're using the apt-get clean command to remove any cached package files, and the rm -rf /var/lib/apt/lists/* command to remove the package lists. These commands remove any unnecessary files and data that may be stored in the intermediate layers.

Using docker builder prune

Let's say you have a Docker image called "myapp" and you've made some changes to your code. When you run the "docker build" command to rebuild the image, Docker will use the cache to speed up the process. However, if you want to rebuild the entire image, including the intermediate layers, you can use the following command to clean the Docker cache:

docker builder prune

This command will remove any cached layers for images that have been removed with "docker rmi" but for which caches are still present. It's important to note that this command only removes cache for images that have been removed with "docker rmi" and are not visible with "docker images --all". The command may take up to 15 minutes to run, so be patient.

For example, let's say you removed the "myapp" image with "docker rmi", but the cached layers are still present. When you try to pull the "myapp" image, you may see the "Already exists" message for some layers. To clean the cache for this image, you can run the following command:

docker builder prune

This will remove any cached layers for the "myapp" image that have been removed with "docker rmi".

Use a third-party tool

There are also several third-party tools that you can use to clean Docker cache. One popular tool is Docker-Clean, which is a simple Python script that can clean up Docker images, containers, volumes, and networks. You can install Docker-Clean using the following command:

pip install docker-clean

Once installed, you can use the following command to clean Docker cache:

docker-clean

This command will remove all the unused images, containers, volumes, and networks.

Conclusion

In conclusion, cleaning Docker cache is an important task that can help you optimize disk usage and improve performance. There are several methods that you can use to clean Docker cache, including using the Docker CLI, adding a Dockerfile instruction, and using a third-party tool. By cleaning Docker cache regularly, you can ensure that your Docker images are lean and efficient, which can help you deliver applications faster and more reliably.