Container as content, from my understanding, it would be utilizing the container itself to deliver the content to the audience. Using the container as a content is kind of a effective and fun way to advertise and most importantly it is very cost saving. It kind of will make you go "Oh my god it is so creative" when you see some really nicely crafted container as content designs. Here are some example I found from Google:
A bus stop turned into a fruit vending machine. It save spaces and really give the opportunity for those who are in a rush to get some vitamins.
A tilted billboard. It doesn't need very complicated copywrite because once you see it, you will understand the message it is trying to to deliver.
A wall with seats and tv which kind save the space.