Many people will find Dock Master a good tool to distributing docks to users. If you would prefer to use Dockutil with Outset and a login-once (or login-every) script, you may run into this issue:
Items are added but not always removed
kcrawford (developer of Dockutil) has a workaround for this:
while ! /usr/local/bin/dockutil --list | grep Messages
# actual dockutil dock modifications here
# /usr/local/bin/dockutil --add ...
I've found that (at least in El Capitan), you can do fractions of seconds in sleep, so
I did quite a bit of experimenting (with the latest Dockutil and the latest El Capitan), and the way I got a login script with Dockutil to work is to