I would like to create a Dock for all users which does not include Siri. It does not appear in the file
com.apple.dock.plist, so where is Siri added to the Dock?
I answered a similar question here: OSX 10.8 - How to keep a custom dock for all users that login
This answer should still apply to macOS Sierra.
Dug up my notes from when I first modified the user template in our base images. Ran back through the process, changing the default Dock to have about 15 items, on a fresh Mac and these steps worked. I did a lot of this from the command line, so as to not fuss with changing/reverting permissions using Finder.
Arrange your Dock however you would like for it to appear on new user accounts, changing the orientation and adding/removing applications.
Browse to ~/Library/Preferences and copy the com.apple.Dock.plist file to your desktop.
Open Terminal and start a root session by typing:
Enter your current account's password when prompted
Copy the copy of your modified dock preference file from your desktop to the User Template in your Mac's System folder.:
cp ~/Desktop/com.apple.Dock.plist /System/Library/User\ Template/English.lproj/Preferences
That should do the trick. I tested this on a fresh Mac by logging into both a Guest account and a new (created after the above steps) standard user account, both accounts showed the proper Dock with 15~ items.
It shouldn't be necessary with these steps but if you run into trouble you should verify that the /System/Library/User Template folder has permissions set to: System - Read/Write & Everyone - No access.