How to use docker buildx debug build command to build and inspect images

Introduction

In this lab, you will learn how to effectively use the docker buildx debug build command to build and inspect Docker images. You will start by building a simple image with default settings using a Dockerfile.

Building upon the basics, you will then explore more advanced build scenarios, including incorporating build arguments and labels into your images. You will also learn how to target specific platforms during the build process and output the resulting image to a file. Finally, the lab will cover techniques for controlling the build cache and securely exposing secrets during the build, providing a comprehensive understanding of the docker buildx debug build command's capabilities.

Skills Graph

Build a simple image with default settings

In this step, you will learn how to build a simple Docker image using a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Docker can build images automatically by reading the instructions from a Dockerfile.

First, navigate to the ~/project directory, which is your working directory for this lab.

cd ~/project

Now, let's create a simple Dockerfile. We will create a file named Dockerfile in the ~/project directory. This Dockerfile will define the steps to create an image based on the ubuntu base image and install the cowsay package.

Use the nano editor to create and edit the Dockerfile:

nano Dockerfile

Add the following content to the Dockerfile:

FROM ubuntu:latest
RUN apt-get update && apt-get install -y cowsay
CMD ["cowsay", "Hello, Docker!"]

Let's break down this Dockerfile:

• FROM ubuntu:latest: This instruction specifies the base image for our new image. We are using the latest version of the official Ubuntu image from Docker Hub.
• RUN apt-get update && apt-get install -y cowsay: This instruction executes commands in a new layer on top of the current image and commits the results. Here, we update the package list and install the cowsay package. The -y flag automatically answers yes to prompts.
• CMD ["cowsay", "Hello, Docker!"]: This instruction provides default commands for an executing container. When a container is started from this image without specifying a command, it will run cowsay "Hello, Docker!".

Save the file by pressing Ctrl + X, then Y, and Enter.

Now that we have our Dockerfile, we can build the image. Use the docker build command. The . at the end of the command tells Docker to look for the Dockerfile in the current directory (~/project). We will also tag the image with a name, for example, my-cowsay-image.

docker build -t my-cowsay-image .

This command will execute the instructions in the Dockerfile step by step. You will see output indicating each step being processed, including downloading the base image (if not already present) and installing cowsay.

After the build process completes, you can verify that the image was created successfully by listing the available images using the docker images command:

docker images

You should see my-cowsay-image listed in the output.

Finally, let's run a container from the image we just built to see if it works as expected.

docker run my-cowsay-image

This command will create and start a new container based on my-cowsay-image. Since we defined a CMD instruction in our Dockerfile, the container will automatically execute cowsay "Hello, Docker!", and you should see the cow saying "Hello, Docker!" in your terminal output.

Build an image with build arguments and labels

In this step, you will learn how to use build arguments (ARG) and labels (LABEL) in your Dockerfile. Build arguments allow you to pass variables to the build process, making your Dockerfile more flexible. Labels are key-value pairs that you can attach to an image to add metadata.

First, make sure you are in the ~/project directory:

cd ~/project

Now, let's modify the Dockerfile we created in the previous step. We will add a build argument to customize the message displayed by cowsay and add a label to the image.

Open the Dockerfile for editing:

nano Dockerfile

Modify the content to include ARG and LABEL instructions:

FROM ubuntu:latest
ARG MESSAGE="Hello, Docker!"
RUN apt-get update && apt-get install -y cowsay
LABEL maintainer="Your Name <[email protected]>"
CMD ["cowsay", "$MESSAGE"]

Let's look at the changes:

• ARG MESSAGE="Hello, Docker!": This instruction defines a build argument named MESSAGE with a default value of "Hello, Docker!". This value can be overridden during the build process.
• LABEL maintainer="Your Name <[email protected]>": This instruction adds a label to the image. Labels are useful for adding information like the maintainer, version, or any other relevant metadata.
• CMD ["cowsay", "$MESSAGE"]: We've changed the CMD instruction to use the value of the MESSAGE build argument.

Save the modified Dockerfile by pressing Ctrl + X, then Y, and Enter.

Now, let's build the image again, but this time we will pass a value for the MESSAGE build argument using the --build-arg flag. We will also give it a new tag, my-custom-cowsay-image.

docker build --build-arg MESSAGE="Building with arguments!" -t my-custom-cowsay-image .

Observe the output during the build process. You should see that the ARG instruction is processed, and the value of MESSAGE is set to "Building with arguments!".

After the build is complete, let's run a container from this new image:

docker run my-custom-cowsay-image

This time, the container should output the custom message "Building with arguments!".

You can also inspect the image to see the labels that were added. Use the docker inspect command followed by the image name:

docker inspect my-custom-cowsay-image

In the output, look for the "Labels" section. You should find the maintainer label with the value you specified in the Dockerfile.

Build an image targeting a specific platform and output to a file

In this step, you will learn how to build a Docker image for a specific platform and how to output the built image to a file. Building for different platforms is useful when you need to deploy your application on systems with different architectures (e.g., ARM vs. x86). Outputting to a file allows you to save the image as a tar archive, which can be easily transferred and loaded on another Docker environment.

First, ensure you are in the ~/project directory:

cd ~/project

We will use the same Dockerfile from the previous step. You can view its content using the cat command:

cat Dockerfile

The output should be:

FROM ubuntu:latest
ARG MESSAGE="Hello, Docker!"
RUN apt-get update && apt-get install -y cowsay
LABEL maintainer="Your Name <[email protected]>"
CMD ["cowsay", "$MESSAGE"]

Now, let's build the image specifically for the linux/arm64 platform. We will use the --platform flag. Note that building for a different platform requires Docker Buildx, which is typically included with recent Docker installations.

docker build --platform linux/arm64 -t my-arm64-cowsay-image .

This command will build the image for the ARM64 architecture. You will see output similar to previous builds, but the build process is targeting a different architecture.

After the build completes, you can list your images to see the newly created image. Notice that the ARCHITECTURE column will show arm64 for this image.

docker images

Next, let's output this image to a tar file. This is useful for saving and sharing images without using a registry. We will use the --output flag with the type=tar option.

docker build --platform linux/arm64 -t my-arm64-cowsay-image . --output type=tar,dest=my-arm64-cowsay-image.tar

This command builds the image again (potentially using the build cache) and then saves it as a tar file named my-arm64-cowsay-image.tar in the ~/project directory.

You can verify that the tar file was created using the ls command:

ls -lh my-arm64-cowsay-image.tar

You should see the file listed with its size. This tar file contains all the layers of the my-arm64-cowsay-image. You could then transfer this file to another machine and load it using docker load -i my-arm64-cowsay-image.tar.

Build an image with cache control and secret exposure

In this step, you will learn about controlling the Docker build cache and how to handle secrets during the build process using BuildKit's secret mount type. The build cache can significantly speed up builds, but sometimes you need to disable it. Handling secrets securely is crucial to avoid embedding sensitive information in your image layers.

First, ensure you are in the ~/project directory:

cd ~/project

Let's modify the Dockerfile to include a step that we might want to control caching for and demonstrate secret handling. We will add a simple file creation step and a step that would hypothetically use a secret.

Open the Dockerfile for editing:

nano Dockerfile

Modify the content to include a RUN command that creates a file with a timestamp and a placeholder for using a secret:

FROM ubuntu:latest
ARG MESSAGE="Hello, Docker!"
RUN apt-get update && apt-get install -y cowsay
LABEL maintainer="Your Name <[email protected]>"
RUN echo "Build time: $(date)" > /app/build_info.txt
## This is a placeholder for a command that would use a secret
## RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret > /app/secret_info.txt
CMD ["cowsay", "$MESSAGE"]

We added RUN echo "Build time: $(date)" > /app/build_info.txt. This command creates a file /app/build_info.txt containing the timestamp of when this layer was built. By default, Docker caches layers. If you build the image multiple times without changing the instructions before this RUN command, this layer might be served from the cache, and the timestamp will not update.

Save the modified Dockerfile by pressing Ctrl + X, then Y, and Enter.

Now, let's build the image with a new tag, my-cached-image.

docker build -t my-cached-image .

Observe the output. If you built the image recently, you might see ---> Using cache for some steps.

To demonstrate cache control, let's build the image again, but this time we will disable the cache for the entire build using the --no-cache flag.

docker build --no-cache -t my-no-cache-image .

You will see that Docker rebuilds every layer, even if the instructions haven't changed. This is useful when you want to ensure that all dependencies are fetched fresh or when a previous build failed in a way that corrupted the cache.

Now, let's discuss handling secrets. The commented-out line ## RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret > /app/secret_info.txt demonstrates how you would use a secret with BuildKit. To use this, you would need to enable BuildKit (which is often enabled by default in recent Docker versions) and provide the secret during the build using the --secret flag.

For example, if you had a file named mysecret.txt containing your secret, you would build like this (this command will not work as is because we don't have a mysecret.txt file and the line is commented out, but it shows the syntax):

## docker build --secret id=mysecret,src=mysecret.txt -t my-secret-image .

The RUN --mount=type=secret,id=mysecret instruction makes the content of the secret available at /run/secrets/mysecret only during the build step. The secret is not included in the final image layers. This is the secure way to handle sensitive information like API keys or passwords during builds.

Since we don't have a secret file and the line is commented out, we will not execute the secret-related build command. However, understanding the concept of --no-cache and the secret mount type is important for controlling your builds and handling sensitive data.

Summary

In this lab, you learned the fundamentals of building Docker images using a Dockerfile. You started by creating a simple Dockerfile based on Ubuntu, installing a package, and defining a default command. This introduced you to the core instructions FROM, RUN, and CMD, and how to build an image using docker build with a tag.

Building upon the basics, you explored more advanced image building techniques. You learned how to incorporate build arguments and labels into your Dockerfile to customize the build process and add metadata to the image. You also practiced building images for specific platforms and outputting the resulting image to a file, demonstrating how to control the build output. Finally, you delved into managing the build cache and securely handling secrets during the build process, highlighting best practices for efficient and secure image creation.