kubectl

$ docker run --rm \
  --network host \
  -v ~/.kube/config:/home/nonroot/.kube/config:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag> \
  cluster-info

List pods in a namespace

Check the status of pods in a specific namespace:

$ docker run --rm \
  --network host \
  -v ~/.kube/config:/home/nonroot/.kube/config:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag> \
  get pods -n default

Apply Kubernetes manifests

Deploy resources to your cluster by mounting a directory with Kubernetes manifests:

$ docker run --rm \
  --network host \
  -v ~/.kube/config:/home/nonroot/.kube/config:ro \
  -v $(pwd)/manifests:/manifests:ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag> \
  apply -f /manifests/

Use as a base image in CI/CD

The kubectl image is commonly used in CI/CD pipelines for deploying applications to Kubernetes:

$ mkdir -p manifests

$ cat > manifests/configmap.yaml << 'EOF'
apiVersion: v1
kind: ConfigMap
metadata:
  name: my-app-config
  namespace: default
data:
  key: value
EOF

$ cat > Dockerfile << 'EOF'
FROM registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag>
COPY manifests/ /manifests/
ENTRYPOINT ["/usr/local/bin/kubectl"]
CMD ["apply", "-f", "/manifests/"]
EOF

$ docker build -t my-kubectl-deploy .

$ docker run --rm \
  --network host \
  -v ~/.kube/config:/home/nonroot/.kube/config:ro \
  my-kubectl-deploy

Non-hardened images vs Ghost hardened images

Key differences

Why no shell or package manager?

Ghost 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 Ghost 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 <container-name>

or mount debugging tools with the Image Mount feature. When using Image Mount, you must override the entrypoint since the default entrypoint is kubectl. Choose a busybox image that matches your base OS:

For Alpine-based images:

$ docker run --rm -it --pid container:<container-name> \
  --entrypoint /dbg/bin/sh \
  --mount=type=image,source=busybox:musl,destination=/dbg,ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag>

For Debian-based images:

$ docker run --rm -it --pid container:<container-name> \
  --entrypoint /dbg/bin/sh \
  --mount=type=image,source=busybox:glibc,destination=/dbg,ro \
  registry.ghost-prod.alphabravo.io/ghost-base/kubectl:<tag>

[!NOTE] The --entrypoint /dbg/bin/sh flag is required because the kubectl image's default entrypoint is /usr/local/bin/kubectl. Without this override, the shell path would be interpreted as a kubectl subcommand.

Image variants

The kubectl Ghost 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 provide FIPS 140-2 compliant cryptographic modules. These images:

• Include OpenSSL with FIPS provider enabled
• Are available in both runtime and dev variants
• Meet compliance requirements for regulated environments

Migrate to a Ghost hardened image

To migrate your application to a Ghost hardened image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Ghost hardened image. This and a few other common changes are listed in the following table of migration notes:

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this is typically going to be an image tagged as dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as dev, your final runtime stage should use a non-dev image variant.

4. Install additional packages

   Ghost hardened images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

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 Ghost 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.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10.

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

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

For the kubectl image, the entrypoint is /usr/local/bin/kubectl. This means any arguments passed to docker run are interpreted as kubectl subcommands. To run a different command, use the --entrypoint flag.