Typically an empty 20 foot shipping container weighs between 1.8-2.2 metric tonnes (about 3,970 - 4,850 lb) and an empty 40 foot shipping container weighs 3.8 - 4.2 tonne (8,340 - 9,260 lb) depending on what kind of container it is. For example, high cube containers tend to be heavier.
Why would these containers be empty?
Yes, especially with the container shortage[1] reported earlier this week, I would be very surprised if empty containers are being sent away from where there already is a shortage.