remove attestation from docker workflow

I don't know what this is or what value this adds, so removing it for now as it doesn't work on gitea

.github/workflows/docker.yml

@@ -51,10 +51,3 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-
-      - name: Generate artifact attestation
-        uses: actions/attest-build-provenance@v3
-        with:
-          subject-name: index.docker.io/ubufugu/dockipelago
-          subject-digest: ${{ steps.push.outputs.digest }}
-          push-to-registry: true