Use full image name

.github/workflows/build.yaml

@@ -36,7 +36,7 @@ jobs:
         with:
           containerfiles: |
             ./Containerfile
-          image: custom-silverblue
+          image: ghcr.io/samhclark/custom-silverblue
           tags: |
             ${{ github.sha }}
             40
@@ -67,11 +67,12 @@ jobs:
         if: ${{ github.event_name != 'pull_request' }}
         env:
           # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
+          IMAGE: ${{ steps.build_image.outputs.image }}
           TAGS: ${{ steps.build_image.outputs.tags }}
           DIGEST: ${{ steps.push.outputs.digest }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+        run: echo "${TAGS}" | xargs -I {} cosign sign --yes ${IMAGE}:{}@${DIGEST}
 
       - name: Echo outputs
         if: github.event_name != 'pull_request'