Building a container image

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I build a container image with my own code in a GitLab CI/CD pipeline?

Objectives

• Know how to build and store a container image in a GitLab CI/CD pipeline

Build a container image

Using container images locally is explained in the CMS Docker lesson. In this lesson, we have used many of them already. We will now see to build a container image including files from your repository in a GitLab CI/CD pipeline.

Here, we expect that you have a Dockerfile in your repository. This file defines what “base” image of your new image and what files goes in it. See the HSF Docker tutorial to learn more about containers.

For the build you can use an existing yml template and extend it in your pipeline. You can do this with include: in your .gitlab-ci.yml.

You will add a job that extends this template with the variables you need:

stages:
  - build
  - test_code

include:
  - project: 'cms-analysis/general/container-image-ci-templates'
    file:
      - 'kaniko-image.gitlab-ci.yml'

build_image:
  extends: .build_kaniko
  stage: build
  variables:
    PUSH_IMAGE: "true"
    REGISTRY_IMAGE_PATH: "${CI_REGISTRY_IMAGE}:${CI_COMMIT_SHORT_SHA}"

code_testing:
  stage: test_code
  script:
    - echo "I'm testing the code"

The variables starting with CI_ are predefined variables for the GitLab CI/CD pipelines, you can find them in the GitLab CI/CD documentation. Here, they are used to get a one-to-one correspondance between the image and the code.

Once the pipeline completes, you will find the container image in the container registry of your repository which you can find in Deploy -> Container registry.

In this manner, you can build a container image even without having docker-engine installed locally. If your repository is public, anyone can use this image.

With snippet above, your container images always have a tag that corresponds to the commit has of your code. If you want to have an image with the latest tag, you can add another tag by defining it in a variable called EXTRA_TAGS in the build_image step:

[...]

build_image:
  extends: .build_kaniko
  stage: build
  variables:
    PUSH_IMAGE: "true"
    REGISTRY_IMAGE_PATH: "${CI_REGISTRY_IMAGE}:${CI_COMMIT_SHORT_SHA}"
    EXTRA_TAGS: "latest"

[...]

You can use this image in GitLab pipelines (or in other workflows):

  image:
    name: gitlab-registry.cern.ch/<your username>/><your repository>:latest

or even use it e.g. on lxplus:

apptainer shell docker://gitlab-registry.cern.ch/<your username>/><your repository>:latest

For specific code versions, you can the commit hash as the tag, instead of :latest.

Key Points

• Easy-to-use yml templates exist for container image building