Replace setup-kustomize action with direct curl installation

- Remove dependency on third-party imranismail/setup-kustomize action
- Use official Kustomize installation script via curl
- Install latest version using the official installation method

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: tsuzu

.github/workflows/docker.yml

@@ -97,11 +97,11 @@ jobs:
         run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
 
       # Generate and upload manifests for tag builds (releases)
-      - name: Setup Kustomize
+      - name: Install Kustomize
         if: startsWith(github.ref, 'refs/tags/')
-        uses: imranismail/setup-kustomize@v1
-        with:
-          kustomize-version: "5.6.0"
+        run: |
+          curl -s "https://raw.githubusercontent.com/kubernetes-sigs/kustomize/master/hack/install_kustomize.sh" | bash
+          sudo mv kustomize /usr/local/bin/
 
       - name: Generate manifests with release image
         if: startsWith(github.ref, 'refs/tags/')