The DocumentRoot points to the folder in the local folder structure where the site is hosted, e.g. (on a linux system) /var/www/html/rockstart.net.
The port number must be included in the VirtualHost definition. Therefore your VHost definition should rather look like:
However, this will still require the users to enter http://rockstart.net:8001 into their browser. I suspect what you had in mind was actually more like a redirection. If the redirection needs to include a change of port (i.e. from 8001 to 80) then you either need to use a reverse proxy or a DNAT firewall.
Or, if the content that is to be made available under http://content.rockstart.net is located on the same server where rockstart.net is located, you could simply use a
redirection within the file system. However, in that case I wonder why you don't just point content.rockstart.net at the same location where rockstart.net:8001 gets its content from?