updatebot: Fix tag format for Vaultwarden

We're using the Alpine variant of the Vaultwarden container images,
since the default variant is significantly larger and we do not need any
of the extra stuff it includes.

updatebot/config.yml

@@ -102,6 +102,7 @@ projects:
   images:
   - name: authelia
     image: ghcr.io/dani-garcia/vaultwarden
+    tag_format: '{version}-alpine'
     source:
       kind: github
       organization: dani-garcia