ci: add Trivy security scanning for source code and Docker image

- Add independent trivy.yml workflow with repo scan and image scan jobs
- Add Trivy image scan step in docker.yml before pushing Docker image
- Add security-events permission for SARIF upload
- Add Trivy Security Scan badge to README

.github/workflows/docker.yml

@@ -10,6 +10,11 @@ on:
     branches:
       - "master"
 
+permissions:
+  contents: read
+  packages: write
+  security-events: write
+
 jobs:
   build-docker:
     runs-on: ubuntu-latest
@@ -59,6 +64,31 @@ jobs:
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
 
+      - name: Build image for scanning
+        uses: docker/build-push-action@v7
+        with:
+          context: .
+          file: docker/Dockerfile
+          platforms: linux/amd64
+          push: false
+          load: true
+          tags: drone-scp:scan
+
+      - name: Run Trivy vulnerability scanner
+        uses: aquasecurity/trivy-action@v0.35.0
+        with:
+          image-ref: "drone-scp:scan"
+          format: "sarif"
+          output: "trivy-image-results.sarif"
+          severity: "CRITICAL,HIGH"
+
+      - name: Upload Trivy scan results to GitHub Security tab
+        uses: github/codeql-action/upload-sarif@v4
+        if: always()
+        with:
+          sarif_file: "trivy-image-results.sarif"
+          category: "trivy-docker-image"
+
       - name: Build and push
         uses: docker/build-push-action@v7
         with: