CI: add debian & gradle build

after discussion on !213, theses changes most certainly would break the debian packages builds (and maybe the gradle one). but I didn't find a dedicated way to ensure that my commits won't break theses use-cases. adding a CI pass to actually do that is reassuring for everyone IMO.

a few notes

• debian-*-tarball is outputting outside of the repo, which is bothering as GitLab CI doesn't clean it
  • adding debian.tarball.root in ant to choose its location
• changes CI trigger to new "rules" syntax, "only" is deprecated
  • FWIW top-level "image" is also deprecated
• lintian (debian's deb linter) is showing a bunch of errors, which doesn't fail the build itself when launched via debuild (it does if directly called)
  • I don't have the knowledge on how to fix most, some are probably breaking; it's worth a look

a few questions

• from the recent changes in "debian-alt" (and deb.i2p.net), look like only Ubuntu's bionic & focal are built, so the CI is only running on theses two
  • it looked weird, did I got it right?
  • can we then remove most subdir of debian-alt? and ant's "{xenial,precise}-release-tarball"?
• ant's debian-binary is not working for me: it tries to call fakeroot debian/rules patch which doesn't exists (so it breaks ant's debian & debianhowto)
  • also, ant's debianrepo exec some unexisting file
  • shall I remove theses targets?
• I didn't understand the difference between debian-tarball and debian-release-tarball, why do we need the first one?