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/networking/network.md

@@ -123,6 +123,40 @@ dokku network:info test-network
 // TODO
 ```
 
+### Routing an app to a known ip:port combination
+
+> New as of 0.25.0
+
+In some cases, it may be necessary to route an app to an existing `$IP:$PORT` combination. This is particularly the case for internal admin tools or services that aren't run by Dokku but have a web ui that would benefit from being exposed by Dokku. This can be done by setting a value for `static-web-lister` and running a few other commands when creating an app.
+
+```shell
+# for a service listening on:
+# - ip address: 127.0.0.1
+# - port: 8080
+
+# create the app
+dokku apps:create local-app
+
+# set the builder to the null builder, which does nothing
+dokku builder:set local-app selected null
+
+# set the scheduler to the null scheduler, which does nothing
+dokku config:set local-app DOKKU_SCHEDULER=null
+
+# set the static-web-listener network property to the ip:port combination for your app.
+dokku network:set local-app static-web-listener 127.0.0.1:8080
+
+# set the port map as desired for the port specified in your static-web-listener
+dokku proxy:ports-set local-app http:80:8080
+
+# set the domains desired
+dokku domains:set local-app local-app.dokku.me
+
+dokku proxy:build-config local-app
+```
+
+Only a single `$IP:$PORT` combination can be routed to for a given app, and that `$IP:$PORT` combination _must_ be accessible to the proxy, or requests to the app may not resolve.
+
 ### Attaching an app to a network
 
 > New as of 0.20.0, Requires Docker 1.21+