Primary motivation for this is speed: GitHub Actions doesn't limit the
number of concurrent jobs to 4, and also provides a docker registry
(GitHub Packages) that we can use to cache the image (building the image
takes cca 5 minutes, fetching it would take less than 10 seconds). This
is done in another commit.
The workflow definition is a bit more complicated because coveralls
support for GitHub Actions is less mature than for Travis CI, so we need
to manually tell coveralls that all parallel builds have finished and
that it can publish the combined result.
