Fix weird nesting with registry name

.github/workflows/build.yaml

@@ -36,7 +36,7 @@ jobs:
         with:
           containerfiles: |
             ./Containerfile
-          image: ghcr.io/samhclark/custom-silverblue
+          image: custom-silverblue
           tags: |
             ${{ github.sha }}
             40
@@ -78,7 +78,7 @@ jobs:
           # SIGSTORE_ID_TOKEN: ${{ GITHUB_TOKEN }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes ${IMAGE}:{}@${DIGEST}
+        run: echo "${TAGS}" | xargs -I {} cosign sign --yes ghcr.io/${{ github.repository_owner }}/${IMAGE}:{}@${DIGEST}
 
       - name: Echo outputs
         if: github.event_name != 'pull_request'