ci: add pnpm/action-setup to docker workflow

setup-node@v5 auto-detects packageManager field and fails without pnpm.

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

Author: theothersideofgod

.github/workflows/docker.yaml

@@ -56,6 +56,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v5
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v6
+
       - name: Setup Node.js
         uses: actions/setup-node@v5
         with: