Fix default docker prefix value of traefik certdumper

docker-compose logs a warning if a environment variable is not set and has no default value.
This adds a default string as the default for traefik certdumper which was added in 7c7b52d9.

tests/build.yml

@@ -31,7 +31,7 @@ services:
     build: ../optional/radicale
 
   traefik-certdumper:
-    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}traefik-certdumper:${MAILU_VERSION:-local}
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}traefik-certdumper:${MAILU_VERSION:-local}
     build: ../optional/traefik-certdumper
 
   admin: