Add composer step to workflow

Author: jaapio

.github/workflows/ci.yml

@@ -9,6 +9,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@master
+      - name: composer
+        uses: docker://composer
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          args: install --no-interaction --prefer-dist --optimize-autoloader
       - name: Build
         uses: elgohr/Publish-Docker-Github-Action@master
         with: