[mod] add a nightly check of the docker images using Trivy

.github/workflows/security.yml

@@ -0,0 +1,27 @@
+name: "Security checks"
+on:
+  schedule:
+    - cron: "05 06 * * *"
+  workflow_dispatch:
+
+jobs:
+  dockers:
+    name: Trivy ${{ matrix.image }}
+    runs-on: ubuntu-20.04
+    strategy:
+      fail-fast: false
+      matrix:
+        image: ["searxng/searxng", "dalf/filtron", "dalf/morty"]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Run Trivy vulnerability scanner
+        uses: aquasecurity/trivy-action@master
+        with:
+          image-ref: '${{ matrix.image }}:latest'
+          format: 'table'
+          exit-code: '1'
+          ignore-unfixed: false
+          vuln-type: 'os,library'
+          severity: 'UNKNOWN,LOW,MEDIUM,HIGH,CRITICAL'