Updated container to run exec commands.

I am writing an app that uses exec to get the current env variables. I have added and successfully used this in my project (which i will be publishing soon to docker)

exec has 2 api commands, therefore i created the 2 functions, and in the example i created you can see the usage.

Please let me know if this is the proper design for the app and if not what needs updating.
Don't like that Id, I know that you were trying to reuse the output from create. But it's better to keep things apart/agnostic from each other.
I would prefer a signature like: Container.prototype.execstart = function(id, opts, callback) {