Clarify binding to $PORT with Dockerfiles

Author: Gabriel Monroy

docs/developer/dockerfile.rst

@@ -12,9 +12,11 @@ own custom Docker image.
 Deploy using Dockerfiles
 ------------------------
 
-With Dockerfiles, the stack you deploy your application upon is limitless. The only
-requirement is that it sets a ENV entry to set the PORT environment variable. This is so
-`slugrunner`_ can listen for when the process is alive or dead. For example:
+With Dockerfiles, the stack you deploy your application upon is limitless. 
+The only requirement is that your application listens on the $PORT environment variable. 
+This is so `slugrunner`_ can bind your application to an available port on the runtime host.
+
+For example:
 
 .. code-block:: console
 
@@ -25,7 +27,7 @@ requirement is that it sets a ENV entry to set the PORT environment variable. Th
     WORKDIR /app
     CMD python -m SimpleHTTPServer $PORT
 
-Which will serve your application's root directory on a static file server on Docker's
-official CentOS image.
+This will serve your application's root directory on a static file server using Docker's
+official CentOS image.  Note the server listens on $PORT, which is defaulted to 8000.
 
 .. _`slugrunner`: https://github.com/deis/slugrunner