Add Trivy Docker image security scan workflow

Scans the Docker image for CRITICAL and HIGH vulnerabilities using Trivy.
Blocks PRs on failures; runs non-blocking on main for visibility. Results
are uploaded to the GitHub Security tab via SARIF.

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

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

@@ -0,0 +1,50 @@
+name: Docker Security Scan
+
+env:
+  DOCKER_IMAGE: katanemo/plano
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+
+      - name: Build Docker Image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/amd64
+          push: false
+          tags: ${{ env.DOCKER_IMAGE }}:scan
+
+      - name: Run Trivy vulnerability scanner
+        uses: aquasecurity/trivy-action@master
+        with:
+          image-ref: ${{ env.DOCKER_IMAGE }}:scan
+          format: table
+          # Fail on PRs so vulnerabilities block merge; on main just report
+          exit-code: ${{ github.event_name == 'pull_request' && '1' || '0' }}
+          severity: CRITICAL,HIGH
+
+      - name: Run Trivy scanner (SARIF for GitHub Security tab)
+        if: always()
+        uses: aquasecurity/trivy-action@master
+        with:
+          image-ref: ${{ env.DOCKER_IMAGE }}:scan
+          format: sarif
+          output: trivy-results.sarif
+          severity: CRITICAL,HIGH
+
+      - name: Upload Trivy results to GitHub Security tab
+        if: always()
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: trivy-results.sarif