Fixes#2802
While it's hard to reproduce the theoretical race is:
1. This goroutine calls ListContainers()
2. Another goroutine removes a container X
3. This goroutine attempts to InspectContainer(X)
However, this bug could be hit in the much simpler case of
InspectContainer() timing out.
In those cases an error is returned and the old code attempted to wrap
the error with the now-nil container.ID. Storing the container ID fixes
that panic.
c47860f928