`latest` is not a valid git tag or revision to get the matching systemd
unit files.
Look up the exact source git commit from the
`'org.opencontainers.image.revision` image config label.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
Add support for installing Docker `master` packages from `moby/moby-bin`
and `dockereng/cli-bin` images.
This could also allow to install arbitrary version from these images but
for now it's only used for `master`.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>