In the process of setting up a Gitlab CI pipeline, I noticed something that I don’t understand.

My .gitlab-ci.yml looks like this:

variables:
  POSTGRES_HOST: "postgres"
  POSTGRES_DB: "postgres"
  POSTGRES_USER: "postgres"
  POSTGRES_PASSWORD: "postgres"
  MIX_ENV: "test"
  
stages:
  - build
  - test

default:
  image: elixir:1.9
  before_script:
    - mix local.hex --force
    - mix local.rebar --force

compile:
  stage: build
  script:
    - apt-get update
    - apt-get install -y postgresql-client
    - mix deps.get --only test
    - mix compile --warnings-as-errors
  artifacts:
    paths:
      - _build
      - deps
    expire_in: 1 week

lint:
  stage: test
  script:
    - mix format --check-formatted

test:
  stage: test
  services:
    - postgres:10.10
  script:
    - mix event_store.setup
    - mix ecto.create
    - mix ecto.migrate
    - mix test

Since jobs are run independently from each other, I save the compilation artifacts from the compile job to avoid recompiling in the next jobs. However, when the test job runs the mix ecto.create command, a compilation is triggered again. Why? Did not the compile job compile everything needed when running mix compile --warnings-as-errors?