probably a silly question. but is there a way to put the pc's shutdown/restart/sleep into one button on the dock??

Yes, it is.

In the first place, a dock does not have "buttons" - it has icons. And no, of course each icon can only represent one item, be that an app, a folder or an internal command. That said, the dock is not really any kind of good place for internal commands, they are much better accommodated in a NextSTART menu.

well depending on what version you are running it could be possible to put them into a sub dock?

Yes, indeed - however, what an extremely clumsy and ugly way of doing things.

I have to confess I always thought it a very bad idea to enable internal commands, as well as system tray and (non-docked) running apps on the dock, although the last of those isn't so bad especially in a scrollable dock as in Ubuntu Linux.