ci: only run Docker job for the main repository (#7980)

.github/workflows/docker.yml

@@ -14,7 +14,7 @@ on:
 
 jobs:
   buildx:
-    if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
+    if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' && github.repository == 'gogs/gogs' }}
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
       cancel-in-progress: true