Log when truncindex.Get returns >1 container

When the user is not using the full has to retrieve a container it's
possible that we find conflicts with the ids of other containers.

At the moment it's just failing saying that it can not find a container,
but it doesn't say why. Adding a small log saying that duplicates where
found is going to help the user.

Closes #8098
Hi @estesp!

Thanks for the feedback. I will change it later today.

This is kinda a "workaround" for now until the proper fix that you mentioned here: https://github.com/docker/docker/issues/8098 is going to be applied. Also, in my comment here https://github.com/docker/docker/issues/8098#issuecomment-61433479 I am mentioning what I think that is your point. The best way to adddress this problem would be to have a unique ways to get container instead error'ing in lots of different places.