Update docker/build-push-action action to v6

| datasource  | package                  | from | to |
| ----------- | ------------------------ | ---- | -- |
| github-tags | docker/build-push-action | v2   | v6 |

Author: renovate[bot]

.github/workflows/forms-backend.yml

@@ -78,7 +78,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push
-        uses: docker/build-push-action@v2
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile