[Feature request] prebuilt testing docker images

Instead of building qemu:docker images locally, we should pull the one built from Travis/Shippable/GitLab by default, and build it only when manually requested.

GitLab has ability to host container images per project and also can build them during CI runs. So I'd suggest that we create GitLab CI jobs that build & publish each of the images under tests/docker on the master branch.

I think this has been done now, so I'm closing this ticket.