Andrew Orr, writing for The Mac Observer:
There is a hidden directory inside of macOS that contains the app icons for built-in services like iCloud Drive. You probably wouldn’t know they are there because you’ll have to enter a specific file path in order to access them. Here’s how to put the iCloud Drive Icon on your dock for easy access to Apple’s cloud storage.
I’ve been moving more and more of my storage into iCloud Drive, so I wanted to make it more easily accessible. I found this post from last year when searching for a way to access iCloud Drive from my menu bar. Putting it in the dock will have to do for now, although the icon isn’t interactive and only acts as a shortcut to open the iCloud Drive folder in Finder.
If you’re in the Finder, the Command-Shift-I keyboard shortcut will open the iCloud Drive folder as well. I can see myself cooking up a Keyboard Maestro macro to create a system-wide shortcut.