Provide seamless support for local.server.port

Previously, the actual HTTP port on which a web application is running on
was only exposed in tests. This commit makes sure to provide that feature
regardless of the environment so that applications can know on which port
they are actually running on.

If there are several containers, each is exposed via the namespace of
their respective application context.

Closes gh-3259

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -400,6 +400,9 @@ code.
 === Use a random unassigned HTTP port
 To scan for a free port (using OS natives to prevent clashes) use `server.port=0`.
 
+TIP: You can know what port got allocated at runtime by looking at the `local.server.port`
+property in the `Environment`.
+
 
 
 [[howto-discover-the-http-port-at-runtime]]