feat: Cache prod _build from docker step

.dockerignore

@@ -1,4 +1,3 @@
-_build
 cover
 .elixir_ls
 *.dump

.gitlab-ci.yml

@@ -28,6 +28,7 @@ fetch_application_dependencies:
     - mix local.hex --force
     - mix local.rebar --force
     - mix deps.get
+    - mix deps.compile
   artifacts:
     paths:
       - deps/phoenix
@@ -82,12 +83,20 @@ build_image_for_commit:
     - master
   services:
     - name: docker:20.10-dind
+  cache:
+    key:
+      files:
+        - mix.lock
+    paths:
+      - _build/
+      - deps/
   script:
     - docker login "https://${CI_REGISTRY}" -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD
     - docker pull $CI_REGISTRY_IMAGE:latest || true
     - export DOCKER_BUILDKIT=1
     - docker build --cache-from $CI_REGISTRY_IMAGE:latest -t $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --build-arg BUILDKIT_INLINE_CACHE=1 .
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
+    - docker cp `docker create $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA`:/root/app/_build/prod _build
 
 deploy_to_tags:
   stage: deploy_tags

Dockerfile

@@ -25,6 +25,7 @@ ADD ./apps/app/mix.exs /root/app/apps/app/
 ADD ./apps/content/mix.exs /root/app/apps/content/
 ADD ./apps/core/mix.exs /root/app/apps/core/
 ADD ./deps /root/app/deps
+ADD ./_build /root/app/_build
 RUN mix deps.get
 
 ADD ./script /root/app/script