You'd need big batteries in the port to act as a buffer and enough energy to charge those in between when no ships are docked. But the ship would be there for quite some time (up to a day or even more). So a few tens/hundreds mw of power would go a long way.

You'd probably want to use a mix of local wind/solar power and a grid connection. Of course, harbors usually already have lots of infrastructure to power heavy industry (steel, refineries, etc.) and transport (e.g. rail). This just adds to that.

There are also other solutions including using container batteries and simply swapping in fresh ones. Which especially in a container harbor shouldn't be that big of a deal.