Add nightly container vuln scan

Introduces a new 'scan' workflow for scanning the main branch container for
vulnerabilities nightly. By default, this will fail for any 'medium' or higher
vulnerability. 

Fixes #613

.github/workflows/scan.yml

@@ -0,0 +1,19 @@
+name: scan
+
+on:
+  schedule:
+    - cron: '0 0 * * *'
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build the container image
+      run: |
+        docker build --tag whoogle-search:test .
+    - name: Initiate grype scan
+      run: |
+        curl -sSfL https://raw.githubusercontent.com/anchore/grype/main/install.sh | sh -s -- -b .
+        chmod +x ./grype
+        ./grype whoogle-search:test --only-fixed