You should be able to click the "Remember my decision for this site" on that page and then it won't ask you again. Or, if you don't want that site to open in that assigned container anymore ... open the Container panel in the upper-right, and then click "Manage Containers" and choose the container mentioned on the "assigned container" page. Then click "Manage site list" and find the site you want to un-assign and click the little trash can icon next to it.