You need to take position:fixed with .container so that it allows you to scroll the page
EDIT
Check it out here http://jsfiddle.net/pmg92/23/ I think this is what you are looking for. I pulled out irrelevant css to not just copy and paste yours. I removed the background rounded image instead of using border-radius , because using the background image, you cannot really compress or expand depending on the content. This will not work in older browsers unless you use web whales and so on.
If you want to use an image, I would suggest slicing the image between the top, bottom and middle. The top will contain the top with radii. The middle will be 1px tall, which will be repeated vertically as needed. The bottom will contain a bottom section with radii.
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