Update CI registry with cairo-specific docker images

This updates .gitlab-ci.yml to use some new Docker images for testing Cairo that I created. The images get generated in https://gitlab.freedesktop.org/federico/cairo-oci-images

The CI runs the tests for the image backend only. Some of them fail, and the way this is presented in gitlab is not ideal: one has to download the CI artifacts to look for which tests failed. I think we may want to make the test runner print out failed tests, instead of just dumping everything to the log file.