`docker create` will not pull an image so we need an additional fallback.
Rework the pull and trust code so it is in one place to facilitate this.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
Also do not require `tar` to be in container, use the standard
image export code that we already have and find the files we
want.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
