Copy docker client structs into v1beta1

Copy docker client structs into our v1beta1 package so we can add yaml tags and fuzz test encode/decode.

Only the last commit is new, the other is part of #672.

This fixes the pod info member not surviving the encode/decode process.
I'm not sure, are we allowed to do that (our boilerplate script will be mad)? I did change the names and added yaml/json tags, so it's not really a direct copy...

I can see a separate file, but I'm not sure a separate package would be good-- presumably it's only going to be for our v1beta1 version and we'll come up with a real solution soon (I hope).