$CI_JOB_IMAGE Not changing between images

GitLab CI/CD
1

Hello,

I have a .gitlab-ci.yml file that looks similar to the following:

cache:
  key: ${CI_JOB_IMAGE}
  paths:
    - _build
    - deps

default:
  before_script:
    - mix local.hex --force
    - mix local.rebar --force
    - mix deps.get

.compile_template: &compile_definition
  stage: build
  script:
    - mix compile --warnings-as-errors

.test_template: &test_definition
  stage: test
  script:
    - mix test --cover

compile:1.10:
  image: elixir:1.10-alpine
  <<: *compile_definition

compile:1.9:
  image: elixir:1.9-alpine
  <<: *compile_definition

test:1.10:
  image: elixir:1.10-alpine
  <<: *test_definition

test:1.9:
  image: elixir:1.9-alpine
  <<: *test_definition

Despite using $CI_JOB_IMAGE as the cache key, they are all pulling from the same cache (labeled -4) and thus failing. Am I using this environment variable correctly? I’ve checked the table of predefined variables there are no others that mention images.

My goal is to have each cache tied to what docker image is being used (thus the version of elixir) to prevent compiling errors.

Thank you!