ci: add jobs to test build docker images on amd64 and arm64

Today merge requests that make changes to the docker images are not fully tested. We need jobs that build (but don't publish) and test all the images, one for amd64 and one for arm64. Basically it's just cd support/docker && ./configure && make test-images).

Such a job will be pretty long, so it should only run when there are changes in tuxmake/runtime/docker.ini or anything under support/docker/, and be skipped in scheduled runs (since those are the ones who publish images, what implies building first).