ci: update dockerfiles to support different knot versions
Example of 2.7 pipeline: https://gitlab.labs.nic.cz/knot/knot-resolver/pipelines/38175
Self sign-up has been disabled due to increased spam activity. If you want to get access, please send an email to a project owner (preferred) or at gitlab(at)nic(dot)cz. We apologize for the inconvenience.
Example of 2.7 pipeline: https://gitlab.labs.nic.cz/knot/knot-resolver/pipelines/38175