Jailer prospers?

Genesis 39:22 says, “And the keeper of the prison committed to Joseph's hand all the prisoners that were in the prison; and whatsoever they did there, he was the doer of it.

39:23 The keeper of the prison looked not to any thing that was under his hand; because the LORD was with him, and that which he did, the LORD made it to prosper.”  How could a jailer prosper?  I can see earlier when he was with the wealthy man how Joseph, a man with no training in anything, could make him prosper but he is in jail, a dungeon.  What possible opportunity could there be in there?