ci: add missing authentication in build image step

.github/workflows/ci.yml

@@ -31,6 +31,14 @@ jobs:
     runs-on: ubuntu-latest
     needs: test
     steps:
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to ghcr.io
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
       - name: Build image
         uses: docker/build-push-action@v2
         with: