I found that the killall Dock solution didn't work (anytime I restarted my computer I had to run the code in Terminal again). While the Alias solution left me wanting to really figure out how to do this!
So, with some searching, here is how I've permanently handled this issue:
Open system preferences
Open software update
Click "advanced..." (bottom right)
Something was clearly borked on the target SSD, as after finding and booting from an external Catalina startup disk, I could reinstall Catalina on the original disk, and all is well. Possibly something with the nested APSF volume chain?
You need at least 20GB to install Catalina (macOS 10.15). Updating this OS probably requires the same amount of space.
I’m guessing that your constant restarts are due to the installing not having enough free space so it’s swapping info from the disk to memory.
Try to free up as much disk space as you can, the more the better. Once you get to 20GB, you ...