fix: validate nginx templates as the running nginx user

The wrapper config used to run `nginx -t` against an app's rendered `nginx.conf` declared no `user` directive, so nginx fell back to its compile-time default and chowned every `proxy_cache_path` directory to that user, including directories that already existed. On Debian and Ubuntu that default is `nobody`, so deploying an app shipping a custom `nginx.conf.sigil` took the running server's live cache directory away for the length of the build and turned every request the cache would serve into an HTTP 500, healing only once the deploy's own reload restored the ownership. The wrapper now declares the same `user` as the host's main nginx config, which is also exposed to custom `validate-config` templates as `$.NGINX_USER`.

plugins/nginx-vhosts/functions

@@ -409,12 +409,6 @@ fn-nginx-vhosts-pre-validate-custom-template() {
 
   local TMP_RENDERED="$TMP_WORK_DIR/nginx.conf"
   local TMP_WRAPPER="$TMP_WORK_DIR/wrapper.conf"
-  local VALIDATE_TEMPLATE="$PLUGIN_AVAILABLE_PATH/nginx-vhosts/templates/validate.conf.sigil"
-  local CUSTOM_VALIDATE_TEMPLATE
-  CUSTOM_VALIDATE_TEMPLATE="$(plugn trigger nginx-app-template-source "$APP" "validate-config")"
-  if [[ -n "$CUSTOM_VALIDATE_TEMPLATE" ]]; then
-    VALIDATE_TEMPLATE="$CUSTOM_VALIDATE_TEMPLATE"
-  fi
 
   dokku_log_info1 "Pre-validating custom nginx.conf.sigil"
 
@@ -424,7 +418,7 @@ fn-nginx-vhosts-pre-validate-custom-template() {
     dokku_log_fail "Custom nginx.conf.sigil failed to render via sigil"
   fi
 
-  sigil -f "$VALIDATE_TEMPLATE" NGINX_CONF="$TMP_RENDERED" | cat -s >"$TMP_WRAPPER"
+  fn-nginx-vhosts-render-validate-wrapper "$APP" "$TMP_RENDERED" "$TMP_WRAPPER"
 
   if ! sudo "$NGINX_LOCATION" -t -c "$TMP_WRAPPER" 2>/dev/null; then
     dokku_log_warn "Failed to validate custom nginx.conf.sigil. Rendered output below..."