Try to publish latest arm docker builds to ghcr.io

This works for Synapse latest:
https://github.com/matrix-org/synapse/pkgs/container/synapse/118785109?tag=latest

I don't know _why_ it works, but I'm noticed some differences between
Synapse and SSync. Let's see if they help.

.github/workflows/docker.yml

@@ -12,7 +12,7 @@ on:
 
 env:
   GHCR_NAMESPACE: matrix-org
-  PLATFORMS: "linux/amd64, linux/arm64"
+  PLATFORMS: "linux/amd64,linux/arm64"
 
 jobs:
   slidingsync:
@@ -39,7 +39,7 @@ jobs:
       - name: Build main sliding sync image
         if: github.ref_name == 'main'
         id: docker_build_sliding_sync
-        uses: docker/build-push-action@v3
+        uses: docker/build-push-action@v4
         with:
           cache-from: type=gha
           cache-to: type=gha,mode=max
@@ -52,7 +52,7 @@ jobs:
       - name: Build release Sliding Sync image
         if: github.event_name == 'release' # Only for GitHub releases
         id: docker_build_sliding_sync_release
-        uses: docker/build-push-action@v3
+        uses: docker/build-push-action@v4
         with:
           cache-from: type=gha
           cache-to: type=gha,mode=max