I have a few posts about using dockutil to manage the macOS dock. dockutil is still a valid and working project, but I'm starting to migrate my scripts to docklib instead, which doesn't seem to require workarounds like this one.
The installation instructions for docklib say you can put the docklib.py file in the same directory as the scripts that invoke it or you can put it "in your Python path." I'd recommend just grabbing the docklib .pkg from the releases page or using the AutoPkg docklib recipes to download it. The .pkg puts docklib.py in /Library/Python/2.7/site-packages/docklib.py.
docklib can be used in an Outset login-once or login-every script. There is no need to explicitly put in a delay to wait for the initial dock to appear before running your script. There is also no need, if you're specifying a dock (rather than modifying an existing one) to remove the default applications Apple puts on the dock. If you're specifying a dock, just say what you want to add. Use this suggested template.