Use system's ephemeral port range for port allocation

Read `/proc/sys/net/ipv4/ip_local_port_range` kernel parameter to obtain
ephemeral port range that now sets the boundaries of port allocator
which finds free host ports for those exported by containers.

Signed-off-by: Michal Minar <miminar@redhat.com>
Upstream-commit: 0eb3544c43cb8e9488d6bf329ceecc11fa0db6f1
Component: engine

components/engine/docs/sources/userguide/usingdocker.md

@@ -154,11 +154,11 @@ ports exposed in our image to our host.
 In this case Docker has exposed port 5000 (the default Python Flask
 port) on port 49155.
 
-Network port bindings are very configurable in Docker. In our last
-example the `-P` flag is a shortcut for `-p 5000` that maps port 5000
-inside the container to a high port (from the range 49153 to 65535) on
-the local Docker host. We can also bind Docker containers to specific
-ports using the `-p` flag, for example:
+Network port bindings are very configurable in Docker. In our last example the
+`-P` flag is a shortcut for `-p 5000` that maps port 5000 inside the container
+to a high port (from *ephemeral port range* which typically ranges from 32768
+to 61000) on the local Docker host. We can also bind Docker containers to
+specific ports using the `-p` flag, for example:
 
     $ sudo docker run -d -p 5000:5000 training/webapp python app.py