CI should validate expected files are included somehow

[tianon]

Related to the typos caught in https://github.com/docker/docker-ce-packaging/pull/914 (that were in my original PR! :joy: :facepalm:), it would probably be a good idea to add to the CI here some kind of validation that the end result includes some fixed set of expected files (such as docker.socket, docker.service, etc), if not outright somehow verifying that they work correctly (maybe installing them in the GHA "host" environment?).