The Dock Police is police force responsible for policing Bigg City Port. Presumably the Dock Police is a type of vessel.
The Dock Police was only mentioned in the TUGS photobook, Warrior's Longest Day. Zorran mocked and frightened a sleepy Izzy Gomez, by putting on an authoritarian voice and pretending to be the Police Dock. Izzy later realized that it was only Zorran, along with Zak and Zebedee, ridiculing him.