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

docs/deployment/builders/null.md

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+# Null Builder
+
+> New as of 0.25.0
+
+The `null` builder does nothing, and is useful for routing to services not managed by Dokku. It should not be used in normal operation. Please see the [network documentation](/docs/networking/network.md#routing-an-app-to-a-known-ip:port-combination) for more information on the aforementioned use case.
+
+## Usage
+
+### Detection
+
+This builder is _never_ auto-detected. The builder _must_  be specified via the `builder:set` command:
+
+```shell
+dokku builder:set node-js-app selected null
+```

docs/deployment/schedulers/null.md

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+# Null Scheduler
+
+> New as of 0.25.0
+
+The `null` scheduler does nothing, and is useful for routing to services not managed by Dokku. It should not be used in normal operation. Please see the [network documentation](/docs/networking/network.md#routing-an-app-to-a-known-ip:port-combination) for more information on the aforementioned use case.
+
+## Usage
+
+### Detection
+
+This scheduler is _never_ auto-detected. The scheduler _must_  be specified via the `config:set` command:
+
+```shell
+dokku config:set node-js-app DOCKER_SCHEDULER=null
+```