Extract test conditions to env for clarity

.github/workflows/docker.yml

@@ -16,6 +16,9 @@ env:
   REGISTRY: ghcr.io
   IMAGE_NAME: ${{ github.repository }}
   TEST_TAG: ${{ github.repository }}:test
+  TEST_EXPECTED_NODE_OUTPUT: "v14.16.1"
+  TEST_EXPECTED_ELIXIR_OUTPUT: "Elixir 1.12.2 (compiled with Erlang/OTP 24)"
+  TEST_EXPECTED_ERLANG_OUTPUT: "Erlang/OTP 24"
 jobs:
   build-and-push-image:
     runs-on: ubuntu-latest
@@ -57,13 +60,13 @@ jobs:
 
       # NOTE: the following tests rely on grep exit code (0 if match found)
       - name: Test that Node can start and has expected version
-        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c 'node --version' | grep 'v14.16.1'
+        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c 'node --version' | grep '${{ env.TEST_EXPECTED_NODE_OUTPUT }}'
 
       - name: Test that Elixir can start and has expected version
-        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c 'elixir --version' | grep 'Elixir 1.12.2 (compiled with Erlang/OTP 24)'
+        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c 'elixir --version' | grep '$${ env.TEST_EXPECTED_ELIXIR_OUTPUT }}'
 
       - name: Test that Erlang can start and has (major) expected version
-        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c "erl -noshell -eval 'erlang:display(erlang:system_info(system_version))' -eval 'init:stop()'" | grep 'Erlang/OTP 24'
+        run: docker run --rm ${{ env.TEST_TAG }} /bin/bash -c "erl -noshell -eval 'erlang:display(erlang:system_info(system_version))' -eval 'init:stop()'" | grep '$${ env.TEST_EXPECTED_ERLANG_OUTPUT }}'
 
       # TODO: handle testing then publication using:
       # - https://github.com/etalab/transport-ops/issues/30