cicd: upgrade images to buster

Note: either combine with (single MR) or do this after #147 (closed), which in turn needs #143 (closed).

The docker targets cicd/docker_targets currently used by the template are based upon Debian stretch. Possibly some older ones use Alpine, though I believe these have been migrated to Debian a while ago.

Change the docker images used to Debian stable. This means we will stop requiring backports for certain packages. It is also likely a newer version of clang{,-{tidy,format}} and possibly others can be used via buster's backports.