workflows: use path context for docker builds

We have to checkout anyway for setvars step, so we can reuse the data instead
of making yet another checkout.

.github/workflows/build.yml

@@ -111,6 +111,7 @@ jobs:
       - name: Build and push
         uses: docker/build-push-action@v3
         with:
+          context: .
           push: ${{ github.event_name == 'release' || (github.event_name == 'workflow_dispatch' && github.event.inputs.push_image == 'true') }}
           platforms: linux/amd64,linux/arm64
           build-args: |