fix: backend build using github token

it seems the default token is no longer valid for building a docker image...?

Author: kkweon

.github/workflows/backend-build.yaml

@@ -79,7 +79,7 @@ jobs:
         with:
           registry: ghcr.io
           username: ${{ github.repository_owner }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          password: ${{ secrets.GHCR_IO_TOKEN }}
 
       - name: Build and push
         uses: docker/build-push-action@v2