<aside> 💡

Kubernetes deprecated Docker as a container runtime in version 1.20 (2020) and removed it in 1.24 (2022). Instead, it now uses Containerd or CRI-O.

✅ The reason: Kubernetes uses the Container Runtime Interface (CRI) to communicate with container runtimes.

🚨 Problem with Docker: Docker does not natively support CRI, so Kubernetes had to use an extra layer called dockershim, which was inefficient.

🔹 What Changed?

• Kubernetes dropped dockershim and directly uses Containerd or CRI-O.
• Docker itself relies on Containerd internally, so nothing really changes for Kubernetes workloads. </aside>

<aside> 💡

Docker itself relies on containerd internally for container execution. So, when Kubernetes removed support for Docker as a container runtime, it wasn't really removing containerd—it was just removing the extra Docker layer (dockershim) that Kubernetes had to use to communicate with it.

</aside>

Even though AKS no longer directly supports Docker as a runtime, Dockerfiles are still widely used for containerized applications. Here’s why:

1. Dockerfile is Just a Standard for Building Images (OCI-Compatible)

• A Dockerfile defines how an image is built, specifying the base image, dependencies, commands, and configurations.
• The output of a docker build command is an OCI (Open Container Initiative) compliant image, which Containerd, CRI-O, and Kubernetes can all run.
• Even though Docker is not the runtime, the images it produces are fully compatible with Kubernetes and AKS.

Example:

Your Dockerfile might look like this:

FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "server.js"]

After building it with docker build -t myapp:latest ., the image can be pushed to Azure Container Registry (ACR) or another registry, then deployed to AKS.

2. Kubernetes & AKS Still Use OCI Images (Which Docker Creates)

Kubernetes (and AKS) do not care how an image was built, only that it conforms to OCI standards.

• The image can be built using Docker, Podman, Buildah, or Kaniko—but the final output is an OCI-compatible container image, which AKS can run with Containerd.

3. Docker is Still the Most Popular Tool for Building Images

• Docker CLI & Docker Desktop are user-friendly and widely adopted.
• CI/CD pipelines (GitHub Actions, Azure DevOps, Jenkins) still use Docker for image building.
• Many developers are familiar with Dockerfile syntax, so switching to an alternative build system would require retraining.