ci: update Docker image reference in GitHub Actions workflow

- Update the Docker image reference from caddy to caddy2 in the GitHub Actions workflow for vulnerability scanning

Signed-off-by: ysicing <i@ysicing.me>

Author: ysicing

.github/workflows/caddy.yml

@@ -56,7 +56,7 @@ jobs:
     - name: Trivy vulnerability scanner caddy
       uses: aquasecurity/trivy-action@master
       with:
-        image-ref: 'ghcr.io/ysicing/caddy'
+        image-ref: 'ghcr.io/ysicing/caddy2'
         format: 'github'
         ignore-unfixed: true
         vuln-type: 'os,library'