In a container I'd go for the newer, shorter types of Knock Outs -- Double Knock Out, Double Pink Knock Out, Sunny Knock Out. They are naturally a bit smaller than the older types making them more amenable to container culture. Do use a top notch potting mix (like CLM) and be prepared to water often in summer. That pot's not that big from a shrub's point of view.

Marty

Thank you! I'm off to research what CLM is.