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).