Use kaniko to build Docker images

Introduced in GitLab 11.2. Requires GitLab Runner 11.2 and above.

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.

Requirements

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.
A Docker config.json file needs to be created with the authentication information for the desired container registry.

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.

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:debug
    entrypoint: [""]
  script:
    - mkdir -p /kaniko/.docker
    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
  rules:
    - if: $CI_COMMIT_TAG

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:

Adding the proxy to /kaniko/.docker/config.json
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
  image:
    name: gcr.io/kaniko-project/executor:debug
    entrypoint: [""]
  script:
    - mkdir -p /kaniko/.docker
    - |-
       KANIKOPROXYBUILDARGS=""
       KANIKOCFG="{ \"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}"
       if [ "x${http_proxy}" != "x" -o "x${https_proxy}" != "x" ]; then
         KANIKOCFG="${KANIKOCFG}, \"proxies\": { \"default\": { \"httpProxy\": \"${http_proxy}\", \"httpsProxy\": \"${https_proxy}\", \"noProxy\": \"${no_proxy}\"}}"
         KANIKOPROXYBUILDARGS="--build-arg http_proxy=${http_proxy} --build-arg https_proxy=${https_proxy} --build-arg no_proxy=${no_proxy}"
       fi
       KANIKOCFG="${KANIKOCFG} }"
       echo "${KANIKOCFG}" > /kaniko/.docker/config.json
    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile $KANIKOPROXYBUILDARGS --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
  only:
    - tags

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:
  - mkdir -p /kaniko/.docker
  - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
  - |
    echo "-----BEGIN CERTIFICATE-----
    ...
    -----END CERTIFICATE-----" >> /kaniko/ssl/certs/additional-ca-cert-bundle.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.