Add documentation for proxy ports scheme handling

Closes #2384

[ci skip]

docs/advanced-usage/persistent-storage.md

@@ -71,7 +71,7 @@ dokku docker-options:add <app> build "-v /tmp/python-test:/opt/test"
 
 You cannot use mounted volumes during the build phase of a Dockerfile deploy. This is because Docker does not support volumes when executing `docker build`.
 
-> NOTE: **This can cause data loss** if you bind a mount under `/app` in buildpack apps as herokuish will attempt to remove the original app path during the build phase.
+> Note: **This can cause data loss** if you bind a mount under `/app` in buildpack apps as herokuish will attempt to remove the original app path during the build phase.
 
 ## Docker-Options Note
 

docs/advanced-usage/proxy-management.md

@@ -139,4 +139,22 @@ dokku proxy:ports-remove node-js-app http:80:5000
 
 By default, buildpack apps and dockerfile apps **without** explicitly exposed ports (i.e. using the `EXPOSE` directive) will be configured with a listener on port `80` (and additionally a listener on 443 if ssl is enabled) that will proxy to the application container on port `5000`. Dockerfile apps **with** explicitly exposed ports will be configured with a listener on each exposed port and will proxy to that same port of the deployed application container.
 
-> NOTE: This default behavior **will not** be automatically changed on subsequent pushes and must be manipulated with the `proxy:ports-*` syntax detailed above.
+> Note: This default behavior **will not** be automatically changed on subsequent pushes and must be manipulated with the `proxy:ports-*` syntax detailed above.
+
+## Proxy Port Scheme
+
+The proxy port scheme is as follows:
+
+- `SCHEME:HOST_PORT:CONTAINER_PORT`
+
+The scheme metadata can be used by proxy implementations in order to properly handle proxying of requests. For example, the built-in `nginx-vhosts` proxy implementation supports both the `http` and `https` schemes.
+
+Developers of proxy implementations are encouraged to use whatever schemes make the most sense, and ignore configurations which they do not support. For instance, a `udp` proxy implementation can safely ignore `http` and `https` port mappings.
+
+To change the proxy implementation in use for an application, use the `proxy:set` command:
+
+```shell
+# no validation will be performed against
+# the specified proxy implementation
+dokku proxy:set node-js-app nginx
+```

docs/configuration/nginx.md

@@ -81,7 +81,7 @@ upstream {{ .APP }} {
 {{ .SSL_SERVER_NAME }}              List of SSL VHOSTS
 ```
 
-> NOTE: Application config variables are available for use in custom templates. To do so, use the form of `{{ var "FOO" }}` to access a variable named `FOO`.
+> Note: Application config variables are available for use in custom templates. To do so, use the form of `{{ var "FOO" }}` to access a variable named `FOO`.
 
 
 ### Example HTTP to HTTPS Custom Template

docs/deployment/methods/dockerfiles.md

@@ -17,7 +17,7 @@ To use a dockerfiles for deployment, commit a valid `Dockerfile` to the root of
 
 Dokku will extract all tcp ports exposed using the `EXPOSE` directive (one port per line) and setup nginx to proxy the same port numbers to listen publicly. If you would like to change the exposed port, you should do so within your `Dockerfile` and app.
 
-> NOTE: Nginx does not support proxying UDP. UDP ports can be exposed by disabling the nginx proxy with `dokku proxy:disable myapp`
+> Note: Nginx does not support proxying UDP. UDP ports can be exposed by disabling the nginx proxy with `dokku proxy:disable myapp`
 
 If you do not explicitly `EXPOSE` a port in your `Dockerfile`, dokku will configure the nginx proxy to listen on port 80 (and 443 for TLS) and forward traffic to your app listening on port 5000 inside the container. Just like buildpack apps, you can also use the `$PORT` environment variable in your app to maintain portability.