It’s probably not worth mentioning now but your cache key being static would mean every build that uses marsvin_mix_install would try to share the same cache. That’s every commit on every branch.

I use the variable $CI_COMMIT_REF_NAME which creates a cache for each branch. A good visual primer is A visual guide to GitLab CI/CD caching to understand the various strategies. A single key may be fine if you don’t branch at all since I think later commits drop earlier jobs.

You can also do print debugging by adding RUN ls -al cache/directory to output contents in the logs. Its hacky as hell and there may be better approaches like setting up a runner locally. You could also do stuff like that in the .gitlab-ci.yml script or before script sections.

Just reviewing your .gitlab-ci.yml. In marsvin_build you have both needs: and dependencies: . This might work just fine, but Gitlab recommends not having both in the same layer: CI/CD YAML syntax reference | GitLab Docs, as one relies on current stage, and the other relies on previous build stage. IDK if that changes anything, but thanks for sharing your scripts.

Yeah, I have a previous step looking like this:

.marsvin:
  image: hexpm/elixir:1.15.4-erlang-26.0.2-debian-bookworm-20230612-slim
  variables:
    MIX_ENV: "test"
    # To have hex and rebar included in cache/artifacts, put MIX_HOME inside
    # the build root.
    MIX_HOME: "${CI_PROJECT_DIR}/packages/marsvin/_mix"

marsvin_mix_install:
  extends: .marsvin
  stage: init
  needs: []
  dependencies: []
  cache:
    key: "marsvin_mix_install"
    paths:
      - packages/marsvin/_mix
      - packages/marsvin/_build
      - packages/marsvin/deps
  before_script:
    - cd packages/marsvin
    - mix local.hex --force --if-missing
    - mix local.rebar --force --if-missing
  script:
    - mix deps.get
    - mix deps.clean --unused
    - mix compile --warnings-as-errors
  artifacts:
    paths:
      - packages/marsvin/_mix
      - packages/marsvin/_build
      - packages/marsvin/deps
    expire_in: 7 days

The exported artifacts are then used in the build step, like this:

marsvin_build:
  extends: .marsvin
  stage: build
  needs:
    - "marsvin_mix_install"
    - "marsvin_mix_test"
  dependencies: ["marsvin_mix_install"]
  image: ${BUILDKIT_IMAGE}
  variables:
    APP_VERSION: "${APP_VERSION}"
    IMAGE_URL_INTERNAL: "${DOCKER_REGISTRY_INTERNAL}/${DOCKER_NS_INT}/marsvin"
  script:
    - cd packages/marsvin
    - ${BUILDKIT_SCRIPT}
      --tag=${IMAGE_URL_INTERNAL}:${IMAGE_VERSION}
      --cache-repo=${IMAGE_URL_INTERNAL}:cache
      --context=.
      --file=Dockerfile

Yeah, I only added that to try to solve aforementioned problem, does not appear to make a difference anyway.