For those that use the online Dock Master webpage to make a com.apple.dock.plist template at:
http://techion.com.au/blog/2015/4/28/dock-master
Just observed a bug in the online Dock Master created com.apple.dock.plist.
In the plist it uses the static-apps and static-others, which causes (tested on High Sierra 10.13.4) the Keep in Dock Option to be removed and the Dock to be immutable.
Using BBEdit (or Textedit) to replace static with persistent (e.g., static-apps -> persistent-apps, static-others -> persistent-others) in the com.apple.dock.plist will bring back the Keep in Dock Option and allows the Dock to be mutable again.
