grant packages:write to docker-image workflow

GITHUB_TOKEN defaults to read-only; without packages:write the push
to ghcr.io/token-for-token/hosted-gateway-frontend gets "installation
not allowed to Create organization package".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/docker-image.yml

@@ -7,6 +7,9 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
 
     steps:
       - name: Checkout