Create golang test coverage artifacts during container build

[zetoph]

As a developer, I want to have a quick overview of the current test coverage.

go test can be extended to provide coverage profiles as files. those can be copied form the build container and added as artifacts to the CI so they can be downloaded after each run.

From there we can use them in multiple ways, e.g. repo badges.

The go test statements are currently encapsulated in the make targets for the respective binaries.