Cache docker build layers

Author: rajadain

.github/workflows/continuous_integration.yml

@@ -25,6 +25,12 @@ jobs:
           aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
           aws-region: ${{ secrets.AWS_REGION }}
 
+      - name: Cache Docker layers
+        uses: actions/cache@v2
+        with:
+          path: ~/.docker
+          key: ${{ runner.os }}-docker-${{ hashFiles('**/Dockerfile')}}
+      
       - name: Set up environment
         run: ./scripts/bootstrap