gitlab-ci: Build and unit test for Alpine Linux edge as well

This adds build and unit test targets for Alpine Linux to the CI. Since I can't maintain this target (lack of C knowledge sadly), these targets are allowed to fail but it'll still be good to catch errors and warnings.

For now it uses the edge repos (rolling release) rather than the stable repos, as 2 required packages (libhandy and polkit-elogind) aren't in the latest release yet. They will however be in Alpine 3.11, so I'll switch the target to that release by then.