Use kaniko to build Docker images

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kaniko is a tool to build container images from a Dockerfile, inside a container or Kubernetes cluster.

kaniko solves two problems with using the Docker-in-Docker build method:

Docker-in-Docker requires privileged mode to function, which is a significant security concern.
Docker-in-Docker generally incurs a performance penalty and can be quite slow.

Prerequisites

To use kaniko with GitLab, a runner with one of the following executors is required:

Building a Docker image with kaniko

When building an image with kaniko and GitLab CI/CD, you should be aware of a few important details:

The kaniko debug image is recommended (gcr.io/kaniko-project/executor:debug) because it has a shell, and a shell is required for an image to be used with GitLab CI/CD.
The entrypoint needs to be overridden, otherwise the build script doesn't run.

In the following example, kaniko is used to:

Build a Docker image.
Then push it to GitLab container registry.

The job runs only when a tag is pushed. A config.json file is created under /kaniko/.docker with the needed GitLab container registry credentials taken from the predefined CI/CD variables GitLab CI/CD provides. These are automatically read by the Kaniko tool.

In the last step, kaniko uses the Dockerfile under the root directory of the project, builds the Docker image and pushes it to the project's container registry while tagging it with the Git tag:

build:
  stage: build
  image:
    name: gcr.io/kaniko-project/executor:v1.23.2-debug
    entrypoint: [""]
  script:
    - /kaniko/executor
      --context "${CI_PROJECT_DIR}"
      --dockerfile "${CI_PROJECT_DIR}/Dockerfile"
      --destination "${CI_REGISTRY_IMAGE}:${CI_COMMIT_TAG}"
  rules:
    - if: $CI_COMMIT_TAG

If you authenticate against the Dependency Proxy, you must add the corresponding CI/CD variables for authentication to the config.json file:

- echo "{\"auths\":{\"${CI_REGISTRY}\":{\"auth\":\"$(printf "%s:%s" "${CI_REGISTRY_USER}" "${CI_REGISTRY_PASSWORD}" | base64 | tr -d '\n')\"},\"$(echo -n $CI_DEPENDENCY_PROXY_SERVER | awk -F[:] '{print $1}')\":{\"auth\":\"$(printf "%s:%s" ${CI_DEPENDENCY_PROXY_USER} "${CI_DEPENDENCY_PROXY_PASSWORD}" | base64 | tr -d '\n')\"}}}" > /kaniko/.docker/config.json

This command strips the port, for example :443, from CI_DEPENDENCY_PROXY_SERVER, so you don't have to include it when referencing images.

Building an image with kaniko behind a proxy

If you use a custom GitLab Runner behind an http(s) proxy, kaniko needs to be set up accordingly. This means:

Passing the http_proxy environment variables as build arguments so the Dockerfile instructions can use the proxy when building the image.

The previous example can be extended as follows:

build:
  stage: build
  variables:
    http_proxy: <your-proxy>
    https_proxy: <your-proxy>
    no_proxy: <your-no-proxy>
  image:
    name: gcr.io/kaniko-project/executor:v1.23.2-debug
    entrypoint: [""]
  script:
    - /kaniko/executor
      --context "${CI_PROJECT_DIR}"
      --build-arg http_proxy=$http_proxy
      --build-arg https_proxy=$https_proxy
      --build-arg no_proxy=$no_proxy
      --dockerfile "${CI_PROJECT_DIR}/Dockerfile"
      --destination "${CI_REGISTRY_IMAGE}:${CI_COMMIT_TAG}"
  rules:
    - if: $CI_COMMIT_TAG

Build a multi-arch image

You can build multi-arch images inside a container by using manifest-tool.

For a detailed guide on how to build a multi-arch image, read Building a multi-arch container image in unprivileged containers.

Using a registry with a custom certificate

When trying to push to a Docker registry that uses a certificate that is signed by a custom CA, you might get the following error:

$ /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --no-push
INFO[0000] Downloading base image registry.gitlab.example.com/group/docker-image
error building image: getting stage builder for stage 0: Get https://registry.gitlab.example.com/v2/: x509: certificate signed by unknown authority

This can be solved by adding your CA's certificate to the kaniko certificate store:

before_script:
  - |
    echo "-----BEGIN CERTIFICATE-----
    ...
    -----END CERTIFICATE-----" >> /kaniko/ssl/certs/ca-certificates.crt

Video walkthrough of a working example

The Least Privilege Container Builds with Kaniko on GitLab video is a walkthrough of the Kaniko Docker Build Guided Exploration project pipeline. It was tested on:

The example can be copied to your own group or instance for testing. More details on what other GitLab CI patterns are demonstrated are available at the project page.

Troubleshooting

403 error: "error checking push permissions"

If you receive this error, it might be due to an outside proxy. Setting the http_proxy and https_proxy environment variables can fix the problem.

Error: kaniko should only be run inside of a container

There is a known incompatibility introduced by Docker Engine 20.10

When the host uses Docker Engine 20.10 or newer, then the gcr.io/kaniko-project/executor:debug image in a version older than v1.9.0 does not work as expected.

When you try to build the image, Kaniko fails with:

kaniko should only be run inside of a container, run with the --force flag if you are sure you want to continue

To resolve this issue, update the gcr.io/kaniko-project/executor:debug container to version at least v1.9.0, for example gcr.io/kaniko-project/executor:v1.23.2-debug.

The opposite configuration (gcr.io/kaniko-project/executor:v1.23.2-debug image and Docker Engine on the host in version 19.06.x or older) works without problems. For the best strategy, you should frequently test and update job environment versions to the newest. This brings new features, improved security and - for this specific case - makes the upgrade on underlying Docker Engine on the runner's host transparent for the job.