Fix: Install docker-compose in CI workflow

This commit addresses an error in the GitHub Actions CI workflow where
`docker-compose: command not found` would occur. The `ubuntu-latest`
runner does not include `docker-compose` by default.

Changes:
- Modified `.github/workflows/elixir.yml`:
  - Added a new step "Set up Docker Compose" in the `test` job.
  - This step runs `sudo apt-get update` and then
    `sudo apt-get install -y docker-compose` to make the
    `docker-compose` command available in the CI environment
    before it is used by subsequent steps.

Author: google-labs-jules[bot]

.github/workflows/elixir.yml

@@ -10,6 +10,11 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v2
 
+    - name: Set up Docker Compose
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y docker-compose
+
     - name: Build Docker image
       run: docker build --build-arg MIX_ENV=test --tag code_comparison:latest .