Whenever a defaults command is run that doesn't specify the exact plist path as you have above, it will only affect the current logged in user account. So defaults write com.apple.dock is actually doing something like
defaults write ~/Library/Preferences/com.apple.dock.plist
To fix this, you can either:
1- Use a Configuration Profile to set the setting, or even set up the Dock and make it immutable at one time. See here for an example.
2- Change the command to capture the logged in user and run the defaults command against their Dock.plist
3- Have the policy run as a login trigger, but substitute the logged in user with $3 in the script.
There are probably other options as well as the ones above.
