I couldn't really figure out containers. I assigned them and got the yellow box and everything, but it doesn't seem to register. I don't think it assigned them correctly, since I didn't really understand what you meant by:
+ objectBody condition
- objectSprite: Set position to objectBody
Did you mean "always - set position"? because if I do that the containers seem to rapidly switch between sprites.
As for your question, construct does seem to detect when a platform object is on collosion with the ground, but only when it changes direction.
Edit: I found a way to work around it - when a platform object is "on the ground", it's Y velocity is set at 100. no need for detectors now, I think!