How to use this image

All examples in this guide use the public image. If you've mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

Add DHI Buildkit as a local builder

Run the following command, replacing <tag> with the desired image variant:

docker buildx create --name buildkit-dhi \
    --driver docker-container \
    --driver-opt image=dhi.io/buildkit:<tag> \
    --use

Troubleshooting

The buildkit image runs as rootless. For more information on how to run and troubleshoot rootless Buildkit, see the Rootless mode documentation⁠

Non-hardened images vs. Docker Hardened Images

Key differences

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug⁠ to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

docker debug <image-name>

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-container \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/docker:<tag> /dbg/bin/sh

Image variants

Docker Hardened Images come in different variants depending on their intended use.

• Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

  • Run as the nonroot user
  • Do not include a shell or a package manager
  • Contain only the minimal set of libraries needed to run the app

• Build-time variants typically include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

  • Run as the root user
  • Include a shell and package manager
  • Are used to build or compile applications

• FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

Migrate to a Docker Hardened Image

Switching to the hardened Docker CLI image is straightforward. You can use it as a drop-in replacement for the standard Docker CLI image in your existing workflows and configurations.

Migration steps

1. Update your image reference. Replace the image reference in your Docker run command or CI/CD configuration:
   • From: docker:cli or docker:latest
   • To: dhi.io/docker:<tag>
2. Ensure the Docker socket is mounted: -v /var/run/docker.sock:/var/run/docker.sock
3. All your existing commands, environment variables, and volume mounts remain the same.

General migration considerations

While the Docker CLI functionality remains the same, be aware of these general differences in Docker Hardened Images:

Troubleshoot migration

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

When using the Docker CLI image, ensure the nonroot user has access to the Docker socket. The socket is typically owned by the docker group (GID 999 or similar). You may need to adjust permissions or run the container with the appropriate group ID.

Docker socket access

If you encounter permission errors when accessing the Docker socket, ensure:

1. The Docker socket is mounted: -v /var/run/docker.sock:/var/run/docker.sock
2. The socket has appropriate permissions (usually 660 or 666)
3. Consider using --group-add to add the container user to the docker group:

$ docker run --rm \
  -v /var/run/docker.sock:/var/run/docker.sock \
  --group-add $(stat -c '%g' /var/run/docker.sock) \
  dhi.io/docker:<tag> ps

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.