Revert GHCR mirror configs to Docker Hub direct pulls

Replace ghcr.io/cartsnitch/mirror/* images with Docker Hub originals,
restore GHCR login guard, and add Docker Hub auth.

Refs: CAR-77

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -52,6 +52,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Log in to GHCR
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
@@ -67,6 +68,12 @@ jobs:
             type=sha,prefix=sha-
             type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Build and push Docker image
         uses: docker/build-push-action@v6
         with: