docker: add some example run commands for debugging a container or having it take over host ports

Dockerfile

@@ -2,8 +2,16 @@
 # see https://www.docker.io
 ###########################
 
+# To build the image:
 # sudo docker.io build -t box .
-# sudo docker.io run -i -t box
+
+# To run a container for testing (with a command prompt and no publicly exposed ports):
+# sudo docker.io run -i -t -P box
+
+# Or to run in the background and expose all of the ports so that the *host* acts as a Mail-in-a-Box:
+# (the SSH port is only available locally, but other ports are exposed publicly and must be available
+# otherwise the container won't start)
+# sudo docker.io run -d -p 22 -p 25:25 -p 53:53/udp -p 443:443 -p 587:587 -p 993:993 box
 
 FROM ubuntu:14.04
 MAINTAINER Joshua Tauberer (http://razor.occams.info)