A previous PR (https://github.com/kubernetes/kubernetes/pull/73726)
added GMSA support to the dockershim. Unfortunately, there was a
bug in there: the registry keys used to pass the cred specs down
to Docker were being cleaned up too early, right after the containers'
creation - before Docker would ever try to read them, when trying to
actually start the container.

This patch fixes this.

An e2e test is also provided in a separate PR.
Signed-off-by: Jean Rouge <rougej+github@gmail.com>