ci: only do final build on main branch

oitherwise there's no tag to pull from

.github/workflows/docker-build.yml

@@ -61,7 +61,6 @@ jobs:
             VERSION=${{ matrix.version }}
           push: true
           platforms: linux/${{ matrix.arch }}
-          # tags: registry.dangerous.tech/dangeroustech/zerotierbridge:latest
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true
 
       - name: Export Digests
@@ -80,6 +79,7 @@ jobs:
 
   MergeRefs:
     name: Do The Horrible Merge Thing
+    if: github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     needs:
       - Docker_Build