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

tests/unit/proxied-app.bats

@@ -0,0 +1,52 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+  create_app
+}
+
+teardown() {
+  destroy_app
+  global_teardown
+}
+
+# covers https://github.com/dokku/dokku/issues/4665
+@test "(proxied-app) default" {
+  run /bin/bash -c "dokku builder:set $TEST_APP selected null"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku config:set $TEST_APP DOKKU_SCHEDULER=null"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku network:set $TEST_APP static-web-listener 127.0.0.1:8080"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku proxy:ports-set $TEST_APP http:80:8080"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku domains:set $TEST_APP $TEST_APP.dokku.me"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku proxy:build-config $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "dokku nginx:show-config $TEST_APP"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains "server 127.0.0.1:8080;"
+}