Add explicit permissions to Docker security scan workflow

Set minimal permissions: contents read for checkout, security-events
write for SARIF upload to the GitHub Security tab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker-security-scan.yml

@@ -9,6 +9,10 @@ on:
       - main
   pull_request:
 
+permissions:
+  contents: read
+  security-events: write
+
 jobs:
   scan:
     runs-on: ubuntu-latest