add docker login

Author: duffney

.github/workflows/attest.yml

@@ -15,6 +15,12 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
       - name: Attest
         uses: actions/attest@v1
         id: attest