Unibuild version and build number

[laeti-tia]

Refering to #21 it would be useful that each different unibuild produced, in the Docker image we provide, had a different version number or at least a different build number. It would help spot the differences between Unibuild runs.

[mfeit-internet2]

Generally, I think this is a good idea. There are a few things to be dealt with:

Unibuild will need a way to be given an identifier for the build and make it available to anything it builds. This would be trivial, and could replace or augment UNIBUILD_TIMESTAMP that's used in non-release builds.

There's no way for Unibuild to influence what ends up on an installed system or in a container. There will have to be a package that grabs the build ID and stores it in an installed file and that every bundle will have to depend on it.

As we build right now, each repo would end up with a unique ID because they're all built separately and then merged together at the end. I would, at some point, like to write a Multibuild that builds the entire project in one go and gives everything the same build ID.

[laeti-tia]

The first place where I think it would be most useful is in the build log, just an echo of the build/version number. And if this number is different for each Unibuild image/arch it is not a problem.

It could then also be used in the packages produced, but this doesn't need to be refactored at the same time.