I don't have any experience of this and know next to nothing about planning, but my guess is that a planning department might well consider the permanent placement of a container to be "development", and I think the decision is pretty much up to them, constrained only by legal precedent of planning appeals. In short, if the planners say it needs permission then it will cost you much more to establish that they are wrong then it would simply to apply for permission. Whether you get it is another matter.
Councils have deemed consent for development that supports their statutory undertaking, so just because a council puts a container on a council managed site without permission doesn't mean you can.