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 to keep it in sync with the CI of phosh. I'll switch it to the stable repos once Alpine Linux 3.11 hits.