What is a container? As I understand it:
You're correct in your impressions of ADT vs data structure- although they're interchangeable to a fairly great extent. An ADT is explicitly abstract, a data structure is a more general term that can be totally abstract, or refer to implementation details as well.
A container is a very abstract idea, but generally it implies (to me anyway) some form of encapsulation- in the sense that it might mask complexities of an underlying object, or possibly present a uniform interface to a range of different possible underlying objects. It's a word so firmly rooted in the programmer vocabulary that people use it all the time, and as such its meaning is very blurry. ;)