.github: don't log in to registries on non-main

Author: Erisa

.github/workflows/docker-build.yml

@@ -16,13 +16,15 @@ jobs:
         uses: docker/setup-buildx-action@v3
       - 
         name: Login to GitHub Registry
+        if: ${{ github.ref == 'refs/heads/main' }}
         uses: docker/login-action@v3
         with:
           registry: ghcr.io
           username: ${{ github.repository_owner }}
           password: ${{ secrets.GITHUB_TOKEN }}
       -
         name: Login to DockerHub
+        if: ${{ github.ref == 'refs/heads/main' }}
         uses: docker/login-action@v3
         with:
           username: erisamoe
@@ -36,4 +38,4 @@ jobs:
           push: ${{ github.ref == 'refs/heads/main' }}
           set: |
             *.cache-from=type=gha
-            *.cache-to=type=gha,mode=max
+            *.cache-to=type=gha,mode=max