CI: add actions:write permission for GHA cache

The build job fails with "buildx failed with: </body></html>" because
the workflow restricts permissions to contents:read only, which sets
actions permission to none. The reusable workflow's cache-to:type=gha
needs actions:write to access the GitHub Actions Cache API.

https://claude.ai/code/session_01WY1ytdnf8Hrx3p7fnmZCUr

Author: claude

.github/workflows/docker.yml

@@ -10,6 +10,7 @@ on:
 
 permissions:
   contents: read
+  actions: write
 
 jobs:
   test: