add platform linux/arm64 to docker/build-push gh action

Author: cioddi

.github/workflows/docker-build.yml

@@ -29,7 +29,7 @@ jobs:
       uses: docker/build-push-action@v5
       with:
         context: .
-        platforms: linux/amd64
+        platforms: linux/amd64,linux/arm64
         push: true
         tags: |
           mapbender/mapbender:latest