I think that the whole interface should be freely (un)dockable. Meaning, there's no dedicated Docker. In my vision you'd enter a "UI arrangement mode", in which you would click on various parts of the interface, and right click and say "dock the selected sections". This way the user would freely create the needed number of "docker" containers, at will. You want transport below the menu bar? OK! You want Piano Roll next to arrangment view? Why not? Etc.