Use buildx in test_merge_requests.yml (#1972)

* Update test_merge_requests.yml

* cache stuff removed

.github/workflows/test_merge_requests.yml

@@ -18,15 +18,22 @@ jobs:
         uses: actions/checkout@v2.3.4
         with:
           submodules: recursive
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1.1.0
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1.3.0
+        id: buildx
       - name: Build image locally
         uses: docker/build-push-action@v2.4.0
         with:
+          builder: ${{ steps.buildx.outputs.name }}
           context: .
           file: ./Dockerfile
           build-args: |
             VCS_REF=${{ github.sha }}
             VCS_VER=${{ github.ref }}
           platforms: linux/amd64,linux/arm/v7,linux/arm64
+          push: false
           load: true
           tags: mailserver-testing:ci
       - name: Run test suite