Add ascii and ceres versions

Why stop at "stable" jessie? Having versions for the "testing" ascii and "unstable" ceres suites might be useful for some.

It should be mostly a matter of adding (parallelized!) build snippets to .gitlab-ci.yml. There may be a few kinks due to changes of course and these should be ironed out in such a way that all three suites build without failure.

This should at least cover the base devuan image but having devuan/builder (and devuan/helper?, see #1 (closed)) images for "unstable" ceres would be beneficial for packaging efforts. Having such images for "testing" ascii is likely not all that useful as packages normally enter via "unstable" ceres.

Of course, "slim" variants (#2 (closed)) would be nice too.