They changed how it works in iOS 11. Swipe up from the bottom to get the dock, (but don’t swipe too far or you get Control Center) then drag the wanted app icon to the side.
I discovered that, too - but what to do if I want to multitask with an app on my iPad, but not in my dock? Isn't that easily possible anymore without having to open the second, not-docked app first? (which I preferably would like to have on the right, not on the left)