rpm: evaluate uses of `%license` macro

[thaJeztah]

We currently only use it in a single package; do we need to do so for other packages, or is it implicit / automatic?

One thing we should also look into; I noticed that we add an explicit %license macro for the Compose plugin docs; https://github.com/docker/docker-ce-packaging/blob/e47d6372bc61efecd4ee9c3ff420d09798b0cad6/rpm/SPECS/docker-compose-plugin.spec#L50-L51

But we don't do this for other packages. It's possible that (given the standard naming used) this happens automatically, but perhaps we should look if additional things should be added.

Originally posted by @thaJeztah in #1112