Skip Docker CI Login/Push on forks (#23564)

.github/workflows/build-image.yml

@@ -33,7 +33,7 @@ jobs:
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
-        if: github.event_name != 'pull_request'
+        if: github.repository == 'mastodon/mastodon' && github.event_name != 'pull_request'
 
       - name: Log in to the Github Container registry
         uses: docker/login-action@v2
@@ -41,7 +41,7 @@ jobs:
           registry: ghcr.io
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
-        if: github.event_name != 'pull_request'
+        if: github.repository == 'mastodon/mastodon' && github.event_name != 'pull_request'
 
       - uses: docker/metadata-action@v4
         id: meta