Building Docker Images with Kaniko

This is the first post in a series about kaniko.

kaniko is a tool to build container images from a Dockerfile, similar to docker build, but without needing a Docker daemon. kaniko builds the images inside a container, executing the Dockerfile commands in userspace, so it allows us to build the images in standard Kubernetes clusters.

This means that in a containerized environment, be it a Kubernetes cluster, a Jenkins agent running in Docker, or any other container scheduler, we no longer need to use Docker in Docker nor do the build in the host system by mounting the Docker socket, simplifying and improving the security of container image builds.

Still, kaniko does not make it safe to run untrusted container image builds, but it relies on the security features of the container runtime. If you have a minimal base image that doesn’t require permissions to unpack, and your Dockerfile doesn’t execute any commands as the root user, you can run Kaniko without root permissions.

kaniko builds the container image inside a container, so it needs a way to get the build context (the directory where the Dockerfile and any other files that we want to copy into the container are) and to push the resulting image to a registry.

The build context can be a compressed tar in a Google Cloud Storage or AWS S3 bucket, a local directory inside the kaniko container, that we need to mount ourselves, or a git repository.

kaniko can be run in Docker, Kubernetes, Google Cloud Build (sending our image build to Google Cloud), or gVisor. gVisor is an OCI sandbox runtime that provides a virtualized container environment. It provides an additional security boundary for our container image builds.

Images can be pushed to any standard Docker registry but also Google GCR and AWS ECR are directly supported.

With Docker daemon image builds (docker build) we have caching. Each layer generated by RUN commands in the Dockerfile is kept and reused if the commands don’t change. In kaniko, because the image builds happen inside a container that is gone after the build we lose anything built locally. To solve this, kaniko can push these intermediate layers resulting from RUN commands to the remote registry when using the --cache flag.

In this series I will be covering using kaniko with several container registries.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s