add a cancel previous runs action

.github/workflows/main.yml

@@ -13,6 +13,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.6.0
+        with:
+          access_token: ${{ github.token }}
       - uses: actions/checkout@v2
       - name: docker build
         run: |