CI/CD overhaul

This MR overhauls our CI/CD pipeline, moving the relevant Dockerfiles into the new knot/knot-resolver-ci repository, which builds the images in its own CI/CD pipeline, removing the need to build the images locally.

The images have been largely cleaned up, removing unnecessary layering, and have also been de-duplicated - different Knot DNS versions are built using the same parent image, which already has all the required dependencies, shortening the build times.

Another advantage is that the images are properly tagged, so we can use separate versions for different branches of Knot Resolver.

The pipeline in that repository is also ready for nightly rebuilds of Knot DNS master, so we can continously test against that (it currently fails, but !1509 (merged) should fix that). ✅