feat: add support for routing an app to a specified host:port

This is useful when there is a service not managed by Dokku but should be exposed via the Dokku routing layer. As an example, some binaries (consul, nomad, vault) expose web uis, and are traditionally run on the host directly vs in a container.

Closes #4665

plugins/nginx-vhosts/functions

@@ -447,10 +447,12 @@ nginx_build_config() {
     if [[ -z "$DOKKU_APP_LISTENERS" ]]; then
       dokku_log_warn_quiet "No web listeners specified for $APP"
     elif (is_deployed "$APP"); then
-      local IMAGE_TAG=$(get_running_image_tag "$APP")
-      local IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG" 2>/dev/null)
-      if ! verify_image "$IMAGE" 2>/dev/null; then
-        dokku_log_fail "Missing image for app"
+      if [[ "$(plugn trigger network-get-static-listeners "$APP" "web")" == "" ]]; then
+        local IMAGE_TAG=$(get_running_image_tag "$APP")
+        local IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG" 2>/dev/null)
+        if ! verify_image "$IMAGE" 2>/dev/null; then
+          dokku_log_fail "Missing image for app"
+        fi
       fi
 
       local NGINX_BUILD_CONFIG_TMP_WORK_DIR=$(mktemp -d "/tmp/dokku-${DOKKU_PID}-${FUNCNAME[0]}.XXXXXX")