Base image for docker releases

[fsimonis]

Please describe the problem you are trying to solve.

Currently, the docker releases of preCICE use ubuntu:22.04 as a base image, which is the latest Ubuntu LTS.

For testing, we may want a release of our baseline though, namely ubuntu:20.04.
This is especially important for the nightly build of preCICE, as it cannot easily be installed using a Debian package.

Describe the solution you propose.

We have 3 options:

1. Provide docker releases based on our Ubuntu baseline ubuntu:20.04
2. Provide docker releases based on the latest Ubuntu LTS ubuntu:22.04 (our current approach)
3. Provide docker releases for both, the baseline and the latest Ubuntu LTS version.

If we choose to provide both, then we need another image. I suggest to keep using precice/precice for the latest LTS, and precice/precice-baseline for the baseline LTS.

Additional context

Question raised here #1949

[MakisH]

Continuing the discussion from #1949 (comment):

For images aimed for releases, then the Ubuntu LTS baseline that we assume as minimum is what we should be using. We are in a transition, so going directly with ubuntu:22.04 is fine.

We should not confuse these images with testing. This can happen either for preCICE via the ci-images and the CI pipeline of preCICE (for platforms where this makes strategic sense), or at a global level at the system tests, by sporadically triggering builds on different bases. We have already tested preCICE v3.0.0 on Ubuntu 20.04 (and an Ubuntu LTS is supposed to be stable anyway).

I don't think that it should be the worry of any bindings/adapter developer to monitor changes in other platforms and I don't think we should publish any such images (e.g., of non-LTS releases) with the argument of testing only.