Also adjust the 'linuxkit/alpine' script to follow the
same pattern.
The new version of the script extract username/password from
the credential helper (or docker) and build and 'expect'
script to feed the info to 'notary'.
They can be invoked by:
DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE="phrase" ./push-manifest.sh ...
Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>
