Use Paketo.io / CloudNativeBuildpacks (CNB) in GitLab CI with Kubernetes executor & unprivileged Runners (without pack CLI & docker) - including Paketo build cache-image and different JDK versions

.gitlab-ci.yml

stages:
  - container-image

variables:
  # see usage of Namespaces at https://docs.gitlab.com/ee/user/group/#namespaces
  REGISTRY_GROUP_PROJECT: $CI_REGISTRY/$CI_PROJECT_NAMESPACE/$CI_PROJECT_NAME

build-publish-image:
  stage: container-image
  # For a detailed description on how to use Paketo.io Buildpacks in GitLab CI
  # with Kubernetes executor & unprivileged Runners (without pack CLI & docker)
  # see https://stackoverflow.com/questions/69569784/use-paketo-io-cloudnativebuildpacks-cnb-in-gitlab-ci-with-kubernetes-executo
  image: paketobuildpacks/builder-jammy-base

  # As we don't have docker available, we can't login to GitLab Container Registry as described in the docs https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#using-the-gitlab-container-registry
  # But we somehow need to access GitLab Container Registry with the Paketo lifecycle
  # So we simply create ~/.docker/config.json as stated in https://stackoverflow.com/a/46422186/4964553
  before_script:
    - mkdir ~/.docker
    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_JOB_TOKEN\"}}}" >> ~/.docker/config.json
    # As described in https://buildpacks.io/docs/for-platform-operators/tutorials/lifecycle/#set-environment-variables
    # we need to set some variables like the platform api version in order to prevent errors like this:
    # failed to get platform API version; please set 'CNB_PLATFORM_API' to specify the desired platform API version
    - export CNB_USER_ID=$(id -u) CNB_GROUP_ID=$(id -g) CNB_PLATFORM_API=0.14
    # Configuring Environment variables for the paketo lifecycle is a bit more sophisticated
    # As described in https://stackoverflow.com/a/79766353/4964553 & the lifecycle spec https://github.com/buildpacks/spec/blob/main/platform.md#user-provided-variables
    # we need to create a single file for each variable, where the file name defines the key and the file contents define the value
    # WE NEED TO USE -n with echo, otherwise we have a trailing newline in the environment variable configuration files, that breaks the parameters usage (https://github.com/paketo-buildpacks/libpak/issues/416)
    - mkdir -p ~/platform/env
    # Using a different JDK than default 21
    - echo -n "25" >> ~/platform/env/BP_JVM_VERSION
    # Using a JDK instead of a JRE at runtime (not recommended for default, but used inside the PMS currently for validation, that relies on Java Bytecode)
    - echo -n "JDK" >> ~/platform/env/BP_JVM_TYPE

  script:
    # We need to add the -platform /home/cnb/platform parameter in order to let the lifecycle use our defined environment variables
    - /cnb/lifecycle/creator -app . -platform /home/cnb/platform -cache-image $REGISTRY_GROUP_PROJECT/paketo-build-cache:latest $REGISTRY_GROUP_PROJECT:latest