Growing leeks in containers

 

Hi, I've been given some leek seeds planted in a tray.

They're grown to about 3" high over the last few weeks so I'll need to
re-pot / plant them soon.

I don't have a a garden, but I do have plenty of space for containers.

What are the best containers to use please? Also, how close should I
plant them and how many to a container?

Thanks a lot,

Tom.