fix: set DOCKER_CONFIG to the directory holding the config.json

plugins/common/functions

@@ -948,13 +948,13 @@ fn-migrate-config-to-property() {
   done
 }
 
-fn-registry-docker-config() {
+fn-registry-docker-config-dir() {
   declare desc="returns docker config file path if per-app registry credentials exist"
   declare APP="$1"
   local config_dir="/var/lib/dokku/config/registry/$APP"
   local config_file="$config_dir/config.json"
 
   if [[ -f "$config_file" ]] && [[ "$(jq -r '.auths | length' "$config_file" 2>/dev/null)" != "0" ]]; then
-    echo "$config_file"
+    echo "$config_dir"
   fi
 }