Add --grant-secret support to docker build

This is useful if you want to use secrets during docker build which will
not be recorded in the final image.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
API docs: add the new secret based requests

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
docs: Add cli docs for the secret store

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)