Track container usage

[praveksharma]

The repository presently contains dockerfiles which have not been updated in a while: centos-7 and centos-8. If these containers are no longer used we should remove them.

It may be worth tracking our container usage across OQS projects (using an automated tool? dependabot?) to ease future maintenance.

[baentsch]

What about the suggestion to document the docker images used by each OQS sub project in its respective top-level PLATFORM.md file (that way also adding credibility to the claims in those files)? And in reverse, mark all CI docker images not referenced in those files as superfluous here?

This approach would also help resolve open-quantum-safe/tsc#2 for good: Any sub project not bearing a PLATFORM.md file is clearly an unsupported/unmaintained sub project (--> One less topic for the TSC to discuss :-)

Beyond the (now inactive as I'm not paid anymore to have interest in :) "RedHat precursor platform" (Centos) there's also an Alpine image I created for "minimal image testing". I'd think it'd be prudent to also review the interest for that (i.e., drop it :) Same thought on Debian Bullseye -- rather add Bookworm -- if someone really wants Debian support (added to a PLATFORM.md file).

What also takes me by surprise is that we're maintaining Ubuntu Focal and Jammy: Why both (used in which CI, respectively)? We arguably should have an Ubuntu:latest instead, that way doing away with specific version names also in all sub project CI configs. That way we don't have to track which Ubuntu version OQS supports but know we're always on the latest LTS (maybe having to scramble and fix stuff as and when a new docker release gets the ":latest" tag :-)