Document Docker port selection on multiple exposed ports

docs/content/providers/docker.md

@@ -117,12 +117,14 @@ When using Docker Compose, labels are specified by the directive
 
 Traefik retrieves the private IP and port of containers from the Docker API.
 
-Port detection works as follows:
+Port detection for private communication works as follows:
 
 - If a container [exposes](https://docs.docker.com/engine/reference/builder/#expose) a single port,
-  then Traefik uses this port for private communication.
+  then Traefik uses this port.
 - If a container [exposes](https://docs.docker.com/engine/reference/builder/#expose) multiple ports,
-  or does not expose any port, then you must manually specify which port Traefik should use for communication
+  then Traefik uses the lowest port.  E.g. if `80` and `8080` are exposed, Traefik will use `80`.
+- If a container does not expose any port, or the selection from multiple ports does not fit,
+  then you must manually specify which port Traefik should use for communication
   by using the label `traefik.http.services.<service_name>.loadbalancer.server.port`
   (Read more on this label in the dedicated section in [routing](../routing/providers/docker.md#port)).
 

docs/content/routing/providers/docker.md

@@ -66,10 +66,11 @@ With Docker, Traefik can leverage labels attached to a container to generate rou
     ```
 
     !!! important "Traefik Connecting to the Wrong Port: `HTTP/502 Gateway Error`"
-        By default, Traefik uses the first exposed port of a container.
+        By default, Traefik uses the lowest exposed port of a container as detailed in
+        [Port Detection](../providers/docker.md#port-detection) of the Docker provider.
 
         Setting the label `traefik.http.services.xxx.loadbalancer.server.port`
-        overrides that behavior.
+        overrides this behavior.
 
 ??? example "Specifying more than one router and service per container"