Cache Composer vendor files

.github/workflows/continuous-integration.yml

@@ -29,6 +29,18 @@ jobs:
       - name: Validate composer.json and composer.lock
         run: composer validate
 
+      - name: Get Composer Cache Directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-composer-
+
       - name: Install dependencies
         uses: ramsey/composer-install@v3
         with: