Fix GitHub Actions Docker build secret passing

Changed from 'secret-files' to 'secrets' for passing GitHub token
to Docker build. The Dockerfile expects the secret as a string value,
not a file path.

This fixes the authentication error when cloning the paladin repository:
- cat: /run/secrets/github_token: No such file or directory
- Authentication failed for paladin.git

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/docker.yml

@@ -63,8 +63,8 @@ jobs:
         ssh: default
         cache-from: type=gha
         cache-to: type=gha,mode=max
-        secret-files: |
-          "github_token=${{ secrets.GITHUB_TOKEN }}"
+        secrets: |
+          github_token=${{ secrets.GITHUB_TOKEN }}
 
     - name: Docker Summary
       run: |