Fix htmlproofer CI job

Dockerhub image got a major update, and we depended on the `latest` tag.
I now pinned us to version 3, because I version 4 doesn't provide
`--external-only`.

Fixes #124.

.woodpecker.yml

@@ -15,7 +15,7 @@ pipeline:
       event: [push, deployment]
 
   htmlproofer:
-    image: olikami/htmlproofer:latest # fork of 18fgsa/html-proofer
+    image: olikami/htmlproofer:3
     commands:
       - apk add git
       - ( git show --no-patch --format=format:%B | grep htmlproofer >/dev/null ) && htmlproofer public/ --external-only --checks-to-ignore ScriptCheck,ImageCheck || true