Dock Island is located in the New World but not much is known about the island. A system of Sea Train lines connects the island to Secon Island. The only noteworthy location on the island is Mobston's docks.
Note: This is a non-canon location that appeared in One Piece Film: Z.
One Piece Film: Z
The Dock Island is situated in the New World. The location’s history is unknown, and it is a non-canon location in the series.
Z has been defeating pirate crews since a year ago, and many of them have been shipwrecked on Dock Island. Even though Mobston repaired their ships each time this occurred, the crews always abandoned their weapons and gear and ended their journey; their resolve and ambitions scrambled. Mobston gathered the abandoned equipment and kept it at his docks to give it to someone with the guts to take on Z.
The Thousand Sunny was severely damaged by the White Tiger and Z's attacks following the Straw Hat Pirates' initial encounter with the Neo Marines, forcing the crew to dock at Dock Island. There, Mobston and his grandchildren instructed Franky on Z and assisted him in fixing the Thousand Sunny.
Mobston also offered to give them the equipment he had kept all year. After defeating Z and convincing Ain to put Nami, Robin, Chopper, and Brook back to their original ages, the Straw Hats (sans Franky) decided to travel to Secon Island.
The Straw Hats returned to Secon Island after Z destroyed it, where they ran into Kuzan. He explained Z's objectives to them and offered them an Eternal Pose that leads to Piriodo. The Straw Hats prepare for combat after Kuzan departs, taking the equipment and weaponry that Mobston had stored.
As Franky completed the Thousand Sunny's repairs, the crew headed to Pirodo. After Z was defeated, the Straw Hats made one final trip to Dock Island to give Mobston his equipment and big goodbye.
Dock Island is located in the New World. It is a non-canon location that was featured in One Piece Film: Z.
The Straw Hat Pirates visited Dock Island after the Thousand Sunny was damaged.
Yes, the One Piece Film: Z was released after the timeskip in the canon story of One Piece series.